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Let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305301.png" />, the group of all invertible <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305302.png" />-matrices over the finite [[Field|field]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305303.png" /> with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305304.png" /> elements and characteristic <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305305.png" />, let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305306.png" /> be the subgroup of all superdiagonal elements, let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305307.png" /> be the subgroup of elements of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305308.png" /> whose diagonal entries are all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s1305309.png" />, and let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053010.png" /> be the subgroup of permutation matrices. In the [[Group algebra|group algebra]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053011.png" /> of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053012.png" /> over any field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053013.png" /> of characteristic <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053014.png" /> or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053015.png" />, the element
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<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053016.png" /></td> </tr></table>
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is an idempotent, called the Steinberg idempotent, and the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053017.png" />-module that it generates in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053018.png" /> by right multiplication is called the Steinberg module (see [[#References|[a8]]]) and is commonly denoted <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053019.png" /> (as are all modules isomorphic to it). A similar construction holds for any [[Finite group|finite group]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053020.png" /> of Lie type (and for any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053022.png" />-pair, which is an axiomatic generalization due to J. Tits) defined over a field of characteristic <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053023.png" /> with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053024.png" /> replaced by a [[Borel subgroup|Borel subgroup]] (which is a certain kind of solvable subgroup), <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053025.png" /> by a maximal unipotent subgroup (cf. [[Unipotent group|Unipotent group]]) of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053026.png" /> (which is also a Sylow <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053027.png" />-subgroup of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053028.png" />; cf. also [[Sylow subgroup|Sylow subgroup]]; [[P-group|<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053029.png" />-group]]) and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053030.png" /> by the corresponding [[Weyl group|Weyl group]]. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053031.png" /> is always irreducible and it has <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053032.png" /> as a basis, so that its dimension is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053033.png" /> (see [[#References|[a8]]]). Its character values are given as follows [[#References|[a3]]]. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053034.png" /> has order prime to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053035.png" />, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053036.png" /> equals, up to a sign which can be determined, the order of a Sylow <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053037.png" />-subgroup of the centralizer of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053038.png" />; otherwise it equals <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053039.png" />.
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Let $G = \operatorname{GL} _ { n } ( \mathbf{F} _ { q } )$, the group of all invertible $( n \times n )$-matrices over the finite [[Field|field]] $\mathbf{F} _ { q }$ with $q$ elements and characteristic $p$, let $B$ be the subgroup of all superdiagonal elements, let $U$ be the subgroup of elements of $B$ whose diagonal entries are all $1$, and let $W$ be the subgroup of permutation matrices. In the [[Group algebra|group algebra]] $k [ G ]$ of $G$ over any field $k$ of characteristic $0$ or $p$, the element
  
In case the characteristic of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053040.png" /> equals <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053041.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053042.png" /> has the following further properties [[#References|[a5]]]. It is the only module (for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053043.png" />) which is both irreducible and projective. As an irreducible module it is the largest (in dimension), and as a projective module it is the smallest since it is a tensor factor of every projective module. It follows that it is also self-dual and that every projective module is also injective and vice versa. Because of these remarkable properties, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053044.png" /> plays a prominent role in ongoing work in the still (2000) unresolved problem of determining all of the irreducible <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053045.png" />-modules (with characteristic <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053046.png" /> still equal to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053047.png" />), or equivalently, as it turns out, of determining all of the irreducible rational <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053048.png" />-modules, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053049.png" /> is the [[Algebraic group|algebraic group]] obtained from <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053050.png" /> by replacing <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053051.png" /> by its algebraic closure <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053052.png" />, i.e., where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053053.png" /> is any simple affine algebraic group of characteristic <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053054.png" /> (see [[#References|[a6]]]). This equivalence comes from the fact that every irreducible <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053055.png" />-module extends to a rational <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053056.png" />-module. In particular, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053057.png" /> extends to the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053058.png" />-module with highest weight <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053059.png" /> times the sum of the fundamental weights, which is accordingly also denoted <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053060.png" />, or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053061.png" /> since there is one such <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053062.png" />-module for each <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053063.png" />. These modules are ubiquitous in the module theory of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053064.png" /> and figure prominently, for example, in the proofs of many cohomological vanishing theorems and in W. Haboush's proof of the [[Mumford hypothesis|Mumford hypothesis]] (see [[#References|[a4]]]).
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\begin{equation*} e = \frac { | U | } { | G | } \left( \sum _ { b \in B } b \right) \left( \sum _ { w \in W } \operatorname { sign } ( w ) w \right) \end{equation*}
  
Back in the finite case, some other constructions of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053065.png" />, with the characteristic of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053066.png" /> now equal to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053067.png" />, are as follows. According to C.W. Curtis [[#References|[a2]]]
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is an idempotent, called the Steinberg idempotent, and the $G$-module that it generates in $k [ G ]$ by right multiplication is called the Steinberg module (see [[#References|[a8]]]) and is commonly denoted $\operatorname{St}$ (as are all modules isomorphic to it). A similar construction holds for any [[Finite group|finite group]] $G$ of Lie type (and for any $B N$-pair, which is an axiomatic generalization due to J. Tits) defined over a field of characteristic $p$ with $B$ replaced by a [[Borel subgroup|Borel subgroup]] (which is a certain kind of solvable subgroup), $U$ by a maximal unipotent subgroup (cf. [[Unipotent group|Unipotent group]]) of $B$ (which is also a Sylow $p$-subgroup of $G$; cf. also [[Sylow subgroup|Sylow subgroup]]; [[P-group|$p$-group]]) and $W$ by the corresponding [[Weyl group|Weyl group]]. $\operatorname{St}$ is always irreducible and it has $\{ e u : u \in U \}$ as a basis, so that its dimension is $| U |$ (see [[#References|[a8]]]). Its character values are given as follows [[#References|[a3]]]. If $x \in G$ has order prime to $p$, then $\chi ( x )$ equals, up to a sign which can be determined, the order of a Sylow $p$-subgroup of the centralizer of $x$; otherwise it equals $0$.
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053068.png" /></td> </tr></table>
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In case the characteristic of $k$ equals $p$, $\operatorname{St}$ has the following further properties [[#References|[a5]]]. It is the only module (for $G$) which is both irreducible and projective. As an irreducible module it is the largest (in dimension), and as a projective module it is the smallest since it is a tensor factor of every projective module. It follows that it is also self-dual and that every projective module is also injective and vice versa. Because of these remarkable properties, $\operatorname{St}$ plays a prominent role in ongoing work in the still (2000) unresolved problem of determining all of the irreducible $G$-modules (with characteristic $k$ still equal to $p$), or equivalently, as it turns out, of determining all of the irreducible rational $\overline { G }$-modules, where $\overline { G }$ is the [[Algebraic group|algebraic group]] obtained from $G$ by replacing $\mathbf{F} _ { q }$ by its algebraic closure $\mathbf{F} _ { q }$, i.e., where $\overline { G }$ is any simple affine algebraic group of characteristic $p$ (see [[#References|[a6]]]). This equivalence comes from the fact that every irreducible $G$-module extends to a rational $\overline { G }$-module. In particular, $\operatorname{St}$ extends to the $\overline { G }$-module with highest weight $q - 1$ times the sum of the fundamental weights, which is accordingly also denoted $\operatorname{St}$, or $\operatorname{St} _ { q }$ since there is one such $\overline { G }$-module for each $q = p , p ^ { 2 } , p ^ { 3 } , . .$. These modules are ubiquitous in the module theory of $\overline { G }$ and figure prominently, for example, in the proofs of many cohomological vanishing theorems and in W. Haboush's proof of the [[Mumford hypothesis|Mumford hypothesis]] (see [[#References|[a4]]]).
  
in which <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053069.png" /> runs through the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053070.png" /> (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053071.png" /> equal to the rank of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053072.png" />) (parabolic) subgroups of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053073.png" /> containing <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053074.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053075.png" /> is the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053076.png" />-module induced by the trivial <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053077.png" />-module, and the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053078.png" /> or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053079.png" /> is used according as the rank <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053080.png" /> of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053081.png" /> is even or odd. For <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053082.png" />, for example, there is one <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053083.png" /> for each solution of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053084.png" /> (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053085.png" />, each <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053086.png" />); it consists of all of the elements of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053087.png" /> that are superdiagonal in the corresponding block matrix form. A third construction, due to L. Solomon and Tits [[#References|[a7]]], yields <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053088.png" /> as the top homology space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053089.png" /> for the Tits simplicial complex or [[Tits building|Tits building]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053090.png" /> of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053091.png" />, formed as follows: corresponding to each parabolic subgroup <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053092.png" /> there exists an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053093.png" />-simplex <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053094.png" /> in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053095.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053096.png" /> is a facet of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053097.png" /> just when <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053098.png" /> contains <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s13053099.png" />. These three constructions are, in fact, closely related to each other (see [[#References|[a9]]]). In particular, the idempotent <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530100.png" /> used at the start can be identified with an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530101.png" />-sphere in the Tits building, the sum over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530102.png" /> corresponding to a decomposition of the sphere into simplexes: in the usual action of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530103.png" /> on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530104.png" /> the reflecting hyperplanes divide <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530105.png" /> into <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530106.png" /> oriented spherical simplexes, each of which is a [[Fundamental domain|fundamental domain]] for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530107.png" />. Finally, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530108.png" /> has a simple presentation (as a linear space). It is generated by the Borel subgroups of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530109.png" /> subject only to the relations that for every parabolic subgroup of rank <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530110.png" /> the sum of the Borel subgroups that it contains is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530111.png" />.
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Back in the finite case, some other constructions of $\operatorname{St}$, with the characteristic of $k$ now equal to $0$, are as follows. According to C.W. Curtis [[#References|[a2]]]
  
There are also infinite-dimensional versions of the above constructions, usually for reductive Lie groups — real, complex or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530112.png" />-adic — such as <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530113.png" />. The <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530114.png" />-adic case most closely resembles the finite case. There, the affine Weyl group and a certain compact-open subgroup, called an Iwahori subgroup, come into play (in place of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530115.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530116.png" />), and the three constructions agree. In [[#References|[a1]]] several types of buildings, Curtis' formula and the Steinberg idempotent, in the guise of a homology cycle, all appear. In the infinite case the constructed object is sometimes called the Steinberg representation, sometimes the special representation.
+
\begin{equation*} \text{St} = \sum _ { P } \pm 1 _ { P } ^ { G }, \end{equation*}
  
References [[#References|[a5]]] and [[#References|[a9]]] are essays on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530117.png" />.
+
in which $P$ runs through the $2 ^ { r }$ ($r$ equal to the rank of $G$) (parabolic) subgroups of $G$ containing $B$, $1  ^{ G } _ { P }$ is the $G$-module induced by the trivial $P$-module, and the $+$ or $-$ is used according as the rank $r _ { P }$ of $P$ is even or odd. For $G = \operatorname{GL} _ { n } ( \mathbf{F} _ { q } )$, for example, there is one $P$ for each solution of $n = a _ { 1 } + \ldots + a _ { s }$ ($1 \leq s \leq n$, each $a _ { i } \geq 1$); it consists of all of the elements of $G$ that are superdiagonal in the corresponding block matrix form. A third construction, due to L. Solomon and Tits [[#References|[a7]]], yields $\operatorname{St}$ as the top homology space $H _ { r - 1 } ( C )$ for the Tits simplicial complex or [[Tits building|Tits building]] $C$ of $G$, formed as follows: corresponding to each parabolic subgroup $P$ there exists an $( r - r _ { P } - 1 )$-simplex $S _ { P }$ in $C$, and $S _ { P }$ is a facet of $S _ { Q }$ just when $P$ contains $Q$. These three constructions are, in fact, closely related to each other (see [[#References|[a9]]]). In particular, the idempotent $e$ used at the start can be identified with an $( r - 1 )$-sphere in the Tits building, the sum over $W$ corresponding to a decomposition of the sphere into simplexes: in the usual action of $W$ on $S ^ { r - 1 } \subset \mathbf{R} ^ { r }$ the reflecting hyperplanes divide $S ^ { r - 1}$ into $| W |$ oriented spherical simplexes, each of which is a [[Fundamental domain|fundamental domain]] for $W$. Finally, $\operatorname{St}$ has a simple presentation (as a linear space). It is generated by the Borel subgroups of $G$ subject only to the relations that for every parabolic subgroup of rank $1$ the sum of the Borel subgroups that it contains is $0$.
 +
 
 +
There are also infinite-dimensional versions of the above constructions, usually for reductive Lie groups — real, complex or $p$-adic — such as $\operatorname { GL} _ { n }$. The $p$-adic case most closely resembles the finite case. There, the affine Weyl group and a certain compact-open subgroup, called an Iwahori subgroup, come into play (in place of $W$ and $B$), and the three constructions agree. In [[#References|[a1]]] several types of buildings, Curtis' formula and the Steinberg idempotent, in the guise of a homology cycle, all appear. In the infinite case the constructed object is sometimes called the Steinberg representation, sometimes the special representation.
 +
 
 +
References [[#References|[a5]]] and [[#References|[a9]]] are essays on $\operatorname{St}$.
  
 
====References====
 
====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top"> A. Borel,   J-P. Serre,   "Cohomologie d'immeubles et de groupes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530118.png" />-arithmétiques" ''Topology'' , '''15''' (1976) pp. 211–232</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top"> C.W. Curtis,   "The Steinberg character of a finite group with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530119.png" />-pair" ''J. Algebra'' , '''4''' (1966) pp. 433–441</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top"> C.W. Curtis,   G.I. Lehrer,   J. Tits,   "Spherical buildings and the character of the Steinberg representation" ''Invent. Math.'' , '''58''' (1980) pp. 201–220</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top"> W. Haboush,   "Reductive groups are geometrically reductive" ''Ann. of Math.'' , '''102''' (1975) pp. 67–83</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top"> J.E. Humphreys,   "The Steinberg representation" ''Bull. Amer. Math. Soc. (N.S.)'' , '''16''' (1987) pp. 237–263</TD></TR><TR><TD valign="top">[a6]</TD> <TD valign="top"> J.C. Jantzen,   "Representations of algebraic groups" , Acad. Press (1987)</TD></TR><TR><TD valign="top">[a7]</TD> <TD valign="top"> L. Solomon,   "The Steinberg character of a finite group with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s130/s130530/s130530120.png" />-pair" , ''Theory of Finite Groups (Harvard Symp.)'' , Benjamin (1969) pp. 213–221</TD></TR><TR><TD valign="top">[a8]</TD> <TD valign="top"> R. Steinberg,   "Prime power representations of finite linear groups II" ''Canad. J. Math.'' , '''9''' (1957) pp. 347–351</TD></TR><TR><TD valign="top">[a9]</TD> <TD valign="top"> R. Steinberg,   "Collected Papers" , Amer. Math. Soc. (1997) pp. 580–586</TD></TR></table>
+
<table>
 +
<tr><td valign="top">[a1]</td> <td valign="top"> A. Borel, J-P. Serre, "Cohomologie d'immeubles et de groupes $S$-arithmétiques" ''Topology'' , '''15''' (1976) pp. 211–232 {{MR|447474}} {{ZBL|}} </td></tr><tr><td valign="top">[a2]</td> <td valign="top"> C.W. Curtis, "The Steinberg character of a finite group with $B N$-pair" ''J. Algebra'' , '''4''' (1966) pp. 433–441 {{MR|201524}} {{ZBL|}} </td></tr><tr><td valign="top">[a3]</td> <td valign="top"> C.W. Curtis, G.I. Lehrer, J. Tits, "Spherical buildings and the character of the Steinberg representation" ''Invent. Math.'' , '''58''' (1980) pp. 201–220 {{MR|0571572}} {{ZBL|0435.20024}} </td></tr>
 +
<tr><td valign="top">[a4]</td> <td valign="top"> W. Haboush, "Reductive groups are geometrically reductive" ''Ann. of Math.'' , '''102''' (1975) pp. 67–83 {{MR|0382294}} {{ZBL|0316.14016}} </td></tr><tr><td valign="top">[a5]</td> <td valign="top"> J.E. Humphreys, "The Steinberg representation" ''Bull. Amer. Math. Soc. (N.S.)'' , '''16''' (1987) pp. 237–263 {{MR|0876960}} {{ZBL|0627.20024}} </td></tr>
 +
<tr><td valign="top">[a6]</td> <td valign="top"> J.C. Jantzen, "Representations of algebraic groups" , Acad. Press (1987) {{MR|0899071}} {{ZBL|0654.20039}} </td></tr><tr><td valign="top">[a7]</td> <td valign="top"> L. Solomon, "The Steinberg character of a finite group with $B N$-pair" , ''Theory of Finite Groups (Harvard Symp.)'' , Benjamin (1969) pp. 213–221 {{MR|}} {{ZBL|}} </td></tr>
 +
<tr><td valign="top">[a8]</td> <td valign="top"> R. Steinberg, "Prime power representations of finite linear groups II" ''Canad. J. Math.'' , '''9''' (1957) pp. 347–351 {{MR|0087659}} {{ZBL|0079.25601}} </td></tr>
 +
<tr><td valign="top">[a9]</td> <td valign="top"> R. Steinberg, "Collected Papers" , Amer. Math. Soc. (1997) pp. 580–586 {{MR|}} {{ZBL|0878.20002}} </td></tr>
 +
</table>

Latest revision as of 13:31, 25 November 2023

Let $G = \operatorname{GL} _ { n } ( \mathbf{F} _ { q } )$, the group of all invertible $( n \times n )$-matrices over the finite field $\mathbf{F} _ { q }$ with $q$ elements and characteristic $p$, let $B$ be the subgroup of all superdiagonal elements, let $U$ be the subgroup of elements of $B$ whose diagonal entries are all $1$, and let $W$ be the subgroup of permutation matrices. In the group algebra $k [ G ]$ of $G$ over any field $k$ of characteristic $0$ or $p$, the element

\begin{equation*} e = \frac { | U | } { | G | } \left( \sum _ { b \in B } b \right) \left( \sum _ { w \in W } \operatorname { sign } ( w ) w \right) \end{equation*}

is an idempotent, called the Steinberg idempotent, and the $G$-module that it generates in $k [ G ]$ by right multiplication is called the Steinberg module (see [a8]) and is commonly denoted $\operatorname{St}$ (as are all modules isomorphic to it). A similar construction holds for any finite group $G$ of Lie type (and for any $B N$-pair, which is an axiomatic generalization due to J. Tits) defined over a field of characteristic $p$ with $B$ replaced by a Borel subgroup (which is a certain kind of solvable subgroup), $U$ by a maximal unipotent subgroup (cf. Unipotent group) of $B$ (which is also a Sylow $p$-subgroup of $G$; cf. also Sylow subgroup; $p$-group) and $W$ by the corresponding Weyl group. $\operatorname{St}$ is always irreducible and it has $\{ e u : u \in U \}$ as a basis, so that its dimension is $| U |$ (see [a8]). Its character values are given as follows [a3]. If $x \in G$ has order prime to $p$, then $\chi ( x )$ equals, up to a sign which can be determined, the order of a Sylow $p$-subgroup of the centralizer of $x$; otherwise it equals $0$.

In case the characteristic of $k$ equals $p$, $\operatorname{St}$ has the following further properties [a5]. It is the only module (for $G$) which is both irreducible and projective. As an irreducible module it is the largest (in dimension), and as a projective module it is the smallest since it is a tensor factor of every projective module. It follows that it is also self-dual and that every projective module is also injective and vice versa. Because of these remarkable properties, $\operatorname{St}$ plays a prominent role in ongoing work in the still (2000) unresolved problem of determining all of the irreducible $G$-modules (with characteristic $k$ still equal to $p$), or equivalently, as it turns out, of determining all of the irreducible rational $\overline { G }$-modules, where $\overline { G }$ is the algebraic group obtained from $G$ by replacing $\mathbf{F} _ { q }$ by its algebraic closure $\mathbf{F} _ { q }$, i.e., where $\overline { G }$ is any simple affine algebraic group of characteristic $p$ (see [a6]). This equivalence comes from the fact that every irreducible $G$-module extends to a rational $\overline { G }$-module. In particular, $\operatorname{St}$ extends to the $\overline { G }$-module with highest weight $q - 1$ times the sum of the fundamental weights, which is accordingly also denoted $\operatorname{St}$, or $\operatorname{St} _ { q }$ since there is one such $\overline { G }$-module for each $q = p , p ^ { 2 } , p ^ { 3 } , . .$. These modules are ubiquitous in the module theory of $\overline { G }$ and figure prominently, for example, in the proofs of many cohomological vanishing theorems and in W. Haboush's proof of the Mumford hypothesis (see [a4]).

Back in the finite case, some other constructions of $\operatorname{St}$, with the characteristic of $k$ now equal to $0$, are as follows. According to C.W. Curtis [a2]

\begin{equation*} \text{St} = \sum _ { P } \pm 1 _ { P } ^ { G }, \end{equation*}

in which $P$ runs through the $2 ^ { r }$ ($r$ equal to the rank of $G$) (parabolic) subgroups of $G$ containing $B$, $1 ^{ G } _ { P }$ is the $G$-module induced by the trivial $P$-module, and the $+$ or $-$ is used according as the rank $r _ { P }$ of $P$ is even or odd. For $G = \operatorname{GL} _ { n } ( \mathbf{F} _ { q } )$, for example, there is one $P$ for each solution of $n = a _ { 1 } + \ldots + a _ { s }$ ($1 \leq s \leq n$, each $a _ { i } \geq 1$); it consists of all of the elements of $G$ that are superdiagonal in the corresponding block matrix form. A third construction, due to L. Solomon and Tits [a7], yields $\operatorname{St}$ as the top homology space $H _ { r - 1 } ( C )$ for the Tits simplicial complex or Tits building $C$ of $G$, formed as follows: corresponding to each parabolic subgroup $P$ there exists an $( r - r _ { P } - 1 )$-simplex $S _ { P }$ in $C$, and $S _ { P }$ is a facet of $S _ { Q }$ just when $P$ contains $Q$. These three constructions are, in fact, closely related to each other (see [a9]). In particular, the idempotent $e$ used at the start can be identified with an $( r - 1 )$-sphere in the Tits building, the sum over $W$ corresponding to a decomposition of the sphere into simplexes: in the usual action of $W$ on $S ^ { r - 1 } \subset \mathbf{R} ^ { r }$ the reflecting hyperplanes divide $S ^ { r - 1}$ into $| W |$ oriented spherical simplexes, each of which is a fundamental domain for $W$. Finally, $\operatorname{St}$ has a simple presentation (as a linear space). It is generated by the Borel subgroups of $G$ subject only to the relations that for every parabolic subgroup of rank $1$ the sum of the Borel subgroups that it contains is $0$.

There are also infinite-dimensional versions of the above constructions, usually for reductive Lie groups — real, complex or $p$-adic — such as $\operatorname { GL} _ { n }$. The $p$-adic case most closely resembles the finite case. There, the affine Weyl group and a certain compact-open subgroup, called an Iwahori subgroup, come into play (in place of $W$ and $B$), and the three constructions agree. In [a1] several types of buildings, Curtis' formula and the Steinberg idempotent, in the guise of a homology cycle, all appear. In the infinite case the constructed object is sometimes called the Steinberg representation, sometimes the special representation.

References [a5] and [a9] are essays on $\operatorname{St}$.

References

[a1] A. Borel, J-P. Serre, "Cohomologie d'immeubles et de groupes $S$-arithmétiques" Topology , 15 (1976) pp. 211–232 MR447474
[a2] C.W. Curtis, "The Steinberg character of a finite group with $B N$-pair" J. Algebra , 4 (1966) pp. 433–441 MR201524
[a3] C.W. Curtis, G.I. Lehrer, J. Tits, "Spherical buildings and the character of the Steinberg representation" Invent. Math. , 58 (1980) pp. 201–220 MR0571572 Zbl 0435.20024
[a4] W. Haboush, "Reductive groups are geometrically reductive" Ann. of Math. , 102 (1975) pp. 67–83 MR0382294 Zbl 0316.14016
[a5] J.E. Humphreys, "The Steinberg representation" Bull. Amer. Math. Soc. (N.S.) , 16 (1987) pp. 237–263 MR0876960 Zbl 0627.20024
[a6] J.C. Jantzen, "Representations of algebraic groups" , Acad. Press (1987) MR0899071 Zbl 0654.20039
[a7] L. Solomon, "The Steinberg character of a finite group with $B N$-pair" , Theory of Finite Groups (Harvard Symp.) , Benjamin (1969) pp. 213–221
[a8] R. Steinberg, "Prime power representations of finite linear groups II" Canad. J. Math. , 9 (1957) pp. 347–351 MR0087659 Zbl 0079.25601
[a9] R. Steinberg, "Collected Papers" , Amer. Math. Soc. (1997) pp. 580–586 Zbl 0878.20002
How to Cite This Entry:
Steinberg module. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Steinberg_module&oldid=18782
This article was adapted from an original article by Robert Steinberg (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article