User:Richard Pinch/sandbox-13

Club

Let $\gamma$ be a limit ordinal. A subset $a \subset \gamma$ is unbounded in $\gamma$ if $\sup a = \gamma$; it is closed in $\gamma$ if it is closed in the order topology on $\gamma$: that is, for any limit $\mu < \gamma$ with $\sup(a\cap\mu) = \mu$ then $\mu \in a$. A club (closed unbounded) subset $a$ is one which is both closed and unbounded in $\gamma$.

If the cofinality $\kappa$ of $\gamma$ is greater than $\omega$, then the intersection of any family of fewer than $\kappa$ clubs is again a club.

References

• Kenneth Kunen, "Set Theory", Studies in Logic 34, College Publications (2013) ISBN 978-1-84890-050-9. Zbl 1262.03001

Weakly compact cardinal

A cardinal $\mathfrak{k}$ is weakly compact if for any 2-colouring of the edges of a complete graph on a vertex set of cardinality $\mathfrak{k}$ there is a homogeneous subgraph of cardinality $\mathfrak{k}$.

Weakly compact cardinals are inaccessible: that is, their existence is independent of ZFC.

Every weakly compact cardinal is a Mahlo cardinal.

References

• Erdős, Paul; Tarski, Alfred "On some problems involving inaccessible cardinals", Essays on the foundations of mathematics, Magnes Press (1961) pp. 50–82, MR0167422. Zbl 0212.32502

Weakly compact operator

An operator $T : X \rightarrow Y$ on Banach spaces is weakly compact if $T$ sends bounded subsets of $X$ into weakly compact subsets of $Y$.

See: Grothendieck space, Dunford–Pettis property, Dunford–Pettis operator.

Grothendieck universe

A type of set in which all of mathematics can be performed. Formally, a set $U$ with the properties:

• $x \in U$ and $y \in x$ implies $y \in U$: that is, $U$ is a transitive set;
• If $x, y \in U$ then the doubleton $\{x,y\} \in U$;
• If $x \in U$ then the power set $\mathcal{P}(x) \in U$;
• If $\{x_\alpha\}_{\alpha \in y}$ is a family of elements of $U$ indexed by $y$, and $y \in U$, then the union $\cup_{\alpha \in y} x_\alpha \in U$.

The empty set is a Grothendieck universe, as is the set $V_\omega$ of hereditarily finite sets.

The existence of a non-trivial Grothendieck universe is equivalent to a large cardinal axiom: the existence of strongly inaccessible cardinals.

References

• Thomas Streicher, "Universes in Toposes" in Crosilla, Laura and Schuster, Peter (edd.) From sets and types to topology and analysis. Towards practicable foundations for constructive mathematics. Clarendon Press (2005). pp. 78–90. ISBN 978-0-19-856651-9 Zbl 1092.03038

Transitive set

A set in which the element relation $\in$ is well-founded. Equivalently a set for which each element is also a subset.

References

• Kenneth Kunen, "Set Theory", Studies in Logic 34, College Publications (2013) ISBN 978-1-84890-050-9. Zbl 1262.03001

Hereditarily finite set

A finite set all of whose elements are finite sets.

Property C

For differential equations, a property of completeness for a certain set of solutions. See

Ordinary differential equations, property C for;

Partial differential equations, property C for.

In measure theory, see Luzin-C-property.

For $C$-sets, see Descriptive set theory.

Finite type

For an algebraic structure, an alternative term for finitely generated.

A sheaf of modules $\def\cF{ {\mathcal F}}\cF$ over a sheaf of rings $\cO$ is a sheaf of finite type if it is locally generated over $\cO$ by a finite number of sections.

A Riemann surface $M$ is of finite type if it can be imbedded in a compact Riemann surface $\tilde M$ such that $\tilde M \setminus M$ consists of finitely many points. Cf. also Riemann surfaces, classification of and (the references to) Double of a Riemann surface.

Local properties of a group

In group theory, if $\mathcal{P}$ is a property of groups, then a group $G$ is said to be locally $\mathcal{P}$ if every finitely-generated subgroup of $G$ has the property $\mathcal{P}$. The term was introduced by A.G. Kurosh.

See

Locally cyclic group

Locally finite group

Locally free group

Locally nilpotent group

Locally solvable group

The term "Locally normal group" does not fit this paradigm.

References

• B. Chandler, W. Magnus, "The History of Combinatorial Group Theory: A Case Study in the History of Ideas", Springer (2012) ISBN 1-4613-9487-2
• A.G. Kurosh, Group theory. (Теория групп) (Russian) OGIZ, Moskva-Leningrad (1944) Zbl 0061.02101

Separable partial order

A partially ordered set $(X,{<})$ is Cantor separable if no strictly increasing linearly ordered subset has a least upper bound: $$ a_1 < a_2 < \cdots < b $$ implies there exists $c$ with $$ a_1 < a_2 < \cdots < c < b \ . $$ It is duBois–Reymond separable if a strictly increasing sequence can be separated from a decreasing sequence of upper bounds: $$ a_1 < a_2 < \cdots < \cdots < b_2 < b_1 $$ implies there exists $c$ with $$ a_1 < a_2 < \cdots < c < \cdots < b_2 < b_1 \ . $$

References

• R.C. Walker, "The Stone–Čech compactification", Springer (1974)

Heron triangle

A triangle for which the lengths of the sides and the area are expressible by integers. Named after Heron (1st century A.D.), who studied triangles with side lengths $13,14,15$ and $5,12,13$, the areas of which are 84 and 30, respectively.

The Pythagorean triangles are special cases (cf. Pythagorean numbers). In this case the area is a congruent number.

The Heron formula for the area $S$ of a triangle in terms of its sides $a$, $b$ and $c$ and semi-perimeter $p=(a+b+c)/2$ is $$S=\sqrt{p(p-a)(p-b)(p-c)},$$ so Heron triangles correspond to integer solutions to $$ s^2 = p(p-a)(p-b)(p-c) \ . $$

Trace

Trace may refer to

• Trace on a field extension
• Trace of a square matrix
• Trace on a C*-algebra
• Trace of a trace-class operator
• An element of a trace monoid
• Reduced trace on a central simple algebra

Trace monoid

Let $A$ be an alphabet with an irreflexive symmetric relation $I$ called independence. The complementary relation $I = A \times A \setminus I$ is the "dependence" relation. Such an alphabet is a concurrence or dependency alphabet. The free monoid on $A$ modulo the relations $ab=ba$ when $a,b \in I$ is the trace monoid on $(A,D)$. The elements of a trace monoid are "traces" and the subsets are the "trace languages".

Trace monoids are used to model concurrency in computer languages.

References

• Diekert, Volker; Rozenberg, Grzegorz (edd) "The Book Of Traces" (World Scientific, 1995) ISBN 981-02-2058-8

Trace-class operator

An operator $T$ on a Hilbert space $H$ with complete orthonormal set $(e_n)$ for which the sum $\sum_n \langle Tx_n , x_n \rangle$ is finite. For such operators, the trace is defined to be the value of this sum. The set of trace-class operators on $H$ coincides with the set of squares of Hilbert-Schmidt operators. The trace-class operators are precisely the Schatten class for $p=1$.

References

• Retherford, J. R. "Hilbert space: Compact operators and the trace theorem" London Mathematical Society Student Texts 27. (Cambridge University Press, 1993) ISBN 0-521-42933-1. Zbl 0783.47031

Schatten class

Schatten ideal

A class of operators on a Hilbert space. Let $T$ be an operator with singular values $\sigma_n$. For $1 \le p < \infty$ we say that $T$ is in the Schatten $p$-class if the sequence $(\sigma_n)$ is in $\ell_p$: that is, if $\sum_n |\sigma_n|^p$ converges, and then the $p$-root of the value is the Schatten $p$-norm of $T$. The Schatten classes form ideals of the operator algebra.

The Schatten $2$-class is precisely the Hilbert–Schmidt operators. The Schatten $1$-class is the trace-class operators.

References

• Retherford, J. R. "Hilbert space: Compact operators and the trace theorem" London Mathematical Society Student Texts 27. (Cambridge University Press, 1993) ISBN 0-521-42933-1. Zbl 0783.47031
• Schatten, Robert. "Norm ideals of completely continuous operators" Ergebnisse der Mathematik und ihrer Grenzgebiete. Neue Folge, 27 . (Springer-Verlag, 1960) ISBN 0090.09402

Singular value

The singular values of a complex matrix $A$ are the eigenvalues of $A^*A$, or equivalently of $AA^*$. The singular value decomposition of $A$ is the expression $A=U\Sigma V$, with $U$ a unitary $(m\times n)$-matrix, $V$ a unitary $(n\times n)$-matrix and $\Sigma$ of the form $$ \Sigma = \begin{pmatrix} {\mathcal D} & 0\\ 0 & 0\end{pmatrix}, $$ where ${\mathcal D}$ is diagonal with entries the singular values $s_1,\dots,s_k$ of $A$ and $k$ the rank of $A$.

In the case of a closed operator $A$ on a Hilbert space, then $A^*$ is a positive operator and the singular values of $A$ are the spectrum of $A^*A$.

Span

Span may refer to

• Linear hull, also called linear span or span
• Span (category theory)

Span (category theory)

A diagram in a category of the form $$ \begin{array}{ccccc} & & C & & \\ & f \swarrow & & \searrow g & \\ A & & & & B \end{array} $$

Two spans with arrows $(f,g)$ and $(f',g')$ are equivalent if for all $D,p,q$ the diagrams $$ \begin{array}{ccccc} & & C & & \\ & f \swarrow & & \searrow g & \\ A & & & & B \\ & p \searrow & & \swarrow q \\ & & D & & \\ \end{array} \ \ \text{and}\ \ \begin{array}{ccccc} & & C & & \\ & f' \swarrow & & \searrow g' & \\ A & & & & B \\ & p \searrow & & \swarrow q \\ & & D & & \\ \end{array} $$ either both commute or both do not commute.

A pushout is the colimit of a span.

References

[1]	S. MacLane, "Categories for the working mathematician" , Springer (1971). ISBN 0-387-98403-8

Triple

Triple

monad, on a category $ \mathfrak R $

A monoid in the category of all endomorphism functors on $ \mathfrak R $. In other words, a triple on a category $ \mathfrak R $ is a covariant functor $ T: \mathfrak R \mathop \rightarrow \limits \mathfrak R $ endowed with natural transformations $ \eta : {\mathop{\rm Id}\nolimits} _ {\mathfrak R} \mathop \rightarrow \limits T $ and $ \mu : T ^ {2} \mathop \rightarrow \limits T $ (here $ {\mathop{\rm Id}\nolimits} _ {\mathfrak R} $ denotes the identity functor on $ \mathfrak R $) such that the following diagrams are commutative:

$$ \begin{array}{crclc} T (X) & \mathop \rightarrow \limits ^ {T ( \eta _ {X} )} &T ^ {2} (X) & \mathop \leftarrow \limits ^ {\eta _ {T (X)}} &T (X) \\ {} &{} _ {1 _ {T (X)}} \searrow &\scriptsize {\mu _ {X}} \downarrow &\swarrow _ {1 _ {T (X)}} &{} \\ {} &{} &T (X) &{} &{} \\ \end{array} $$

$$ \begin{array}{rcl} T ^ {3} (X) & \mathop \rightarrow \limits ^ {T ( \mu _ {X} )} &T ^ {2} (X) \\ \scriptsize {\mu _ {T (X)}} \downarrow &{} &\downarrow \scriptsize {\mu _ {X}} \\ T ^ {2} (X) & \mathop \rightarrow \limits _ {\mu _ {X}} &T (X) \\ \end{array} $$

A triple is sometimes called a standard construction, cf. [2].

For any pair of adjoint functors $ F : \mathfrak R \mathop \rightarrow \limits \mathfrak L $ and $ G: \mathfrak L \mathop \rightarrow \limits \mathfrak R $ (see Adjoint functor) with unit and co-unit of adjunction $ \eta : {\mathop{\rm Id}\nolimits} _ {\mathfrak R} \mathop \rightarrow \limits GF $ and $ \epsilon : FG \mathop \rightarrow \limits {\mathop{\rm Id}\nolimits} _ {\mathfrak R} $, respectively, the functor $ T = GF: \mathfrak R \mathop \rightarrow \limits \mathfrak R $ endowed with $ \eta : {\mathop{\rm Id}\nolimits} _ {\mathfrak R} \mathop \rightarrow \limits T $ and $ \mu = G ( \epsilon _ {F} ): T ^ {2} \mathop \rightarrow \limits T $ is a triple on $ \mathfrak R $. Conversely, for any triple $ (T, \eta , \mu ) $ there exist pairs of adjoint functors $ F $ and $ G $ such that $ T = GF $, and the transformations $ \eta $ and $ \mu $ are obtained from the unit and co-unit of the adjunction in the manner described above. The different such decompositions of a triple may form a proper class. In this class there is a smallest element (the Kleisli construction) and a largest element (the Eilenberg–Moore construction).

Examples.

1) In the category of sets, the functor which sends an arbitrary set to the set of all its subsets has the structure of a triple. Each set $ X $ is naturally imbedded in the set of its subsets via singleton sets, and to each set of subsets of $ X $ one associates the union of these subsets.

2) In the category of sets, every representable functor $ H _ {A} (X) = H (A, X) $ carries a triple: The mapping $ \eta _ {X} : X \mathop \rightarrow \limits H (A, X) $ associates to each $ x \in X $ the constant function $ f _ {x} : A \mathop \rightarrow \limits X $ with value $ x $; the mapping $ \mu _ {X} : H (A, H (A, X)) \simeq H (A \times A, X) \mathop \rightarrow \limits H (A, X) $ associates to each function of two variables its restriction to the diagonal.

3) In the category of topological spaces, each topological group $ G $, with unit $ e $, enables one to define a functor $ T _ {G} (X) = X \times G $ that carries a triple: Each element $ x \in X $ is taken to the element $ (x, e) $ and the mapping $ \mu : X \times G \times G \mathop \rightarrow \limits X \times G $ is defined by $ \mu _ {X} (x, g, g ^ \prime ) = (x, gg ^ \prime ) $.

4) In the category of modules over a commutative ring $ R $, each (associative, unital) $ R $- algebra $ A $ gives rise to a triple structure on the functor $ T _ {A} (X) = X \otimes _ {R} A $, in a manner similar to Example 3).

References

[1]	J.F. Adams, "Infinite loop spaces" , Princeton Univ. Press (1978)
[2]	R. Godement, "Topologie algébrique et théorie des faisceaux" , Hermann (1958)
[3]	M.Sh. Tsalenko, E.G. Shul'geifer, "Categories" J. Soviet Math. , 7 : 4 (1977) pp. 532–586 Itogi Nauk. i Tekhn. Algebra Topol. Geom. , 13 (1975) pp. 51–148
[4]	S. MacLane, "Categories for the working mathematician" , Springer (1971)
[5]	E.G. Manes, "Algebraic theories" , Springer (1976)

Comments

The non-descriptive name "triple" for this concept has now largely been superseded by "monad" , although there is an obstinate minority of category-theorists who continue to use it. A comonad (or cotriple) on a category $ \mathfrak R $ is a monad on $ \mathfrak R ^ {op} $; in other words, it is a functor $ T: \mathfrak R \mathop \rightarrow \limits \mathfrak R $ equipped with natural transformations $ \epsilon : T \mathop \rightarrow \limits {\mathop{\rm Id}\nolimits} _ {\mathfrak R} $ and $ \delta : T \mathop \rightarrow \limits T ^ {2} $ satisfying the duals of the commutative diagrams above. Every adjoint pair of functors ( $ F \dashv G $) gives rise to a comonad structure on the composite $ FG $, as well as a monad structure on $ GF $.

An important example of a functor which carries a comonad structure is $ \Lambda : {\mathop{\rm Ring}\nolimits} \mathop \rightarrow \limits {\mathop{\rm Ring}\nolimits} $, $ \Lambda (A)=1+tA[[t]] $, or, equivalently, the functor of big Witt vectors, cf. $ \lambda $- ring; Witt vector. A special case of the natural transformation $ W(A) \mathop \rightarrow \limits \Lambda (W(A)) $ occurs in algebraic number theory as the Artin–Hasse exponential, [a5].

Monads in the category of sets can be equivalently described by sets $ T(n) $ of $ n $- ary operations for each cardinal number (or set) $ n $; $ \eta _ {n} : n \mathop \rightarrow \limits T(n) $ gives the projection operations $ (x _ {1} , x _ {2} ,\dots) \mapsto x _ {i} $, and $ \mu $ gives the rules for composing operations. See [5] or [a1]. This approach extends to monads in arbitrary categories, but it has not proved useful in general, as it has in or near sets.

Of the two canonical ways of constructing an adjunction from a given monad, mentioned in the main article above, the Eilenberg–Moore construction (or category of $ T $- algebras) is by far the more important. Given a monad $ (T, \eta , \mu ) $ on a category $ \mathfrak R $, a $ T $- algebra in $ \mathfrak R $ is a pair $ (A, \alpha ) $ where $ \alpha : TA \mathop \rightarrow \limits A $ is a morphism such that

$$ \begin{array}{lcr} A \mathop \rightarrow \limits ^ {\eta _ {A}} &TA & \mathop \leftarrow \limits ^ {\mu _ {A}} T ^ {2} A \\ {} _ {1 _ {A}} \nwarrow &\scriptsize \alpha \downarrow &\downarrow \scriptsize {T _ {A}} \\ {} & A & \mathop \leftarrow \limits _ \alpha TA \\ \end{array} $$

commutes. A homomorphism of $ T $- algebras $ (A, \alpha ) \mathop \rightarrow \limits (B, \beta ) $ is a morphism $ f: A \mathop \rightarrow \limits B $ in $ \mathfrak R $ such that

$$ \begin{array}{rcl} TA & \mathop \rightarrow \limits ^ {Tf} &TB \\ \scriptsize \alpha \downarrow &{} &\downarrow \scriptsize \beta \\ A &\mathop \rightarrow \limits _ {f} & B \\ \end{array} $$

commutes; thus, one has a category $ \mathfrak R ^ {T} $ of $ T $- algebras, with an evident forgetful functor $ G ^ {T} : \mathfrak R ^ {T} \mathop \rightarrow \limits \mathfrak R $. The functor $ G ^ {T} $ has a left adjoint $ F ^ { T} $, which sends an object $ A $ of $ \mathfrak R $ to the $ T $- algebra $ (TA, \mu _ {A} ) $, and the monad induced by the adjunction ( $ F ^ { T} \dashv G ^ {T} $) is the one originally given.

Now the Kleisli category of $ (T, \eta , \mu ) $ is just the full subcategory of $ \mathfrak R ^ {T} $ on the objects $ F ^ { T} (A) $: the category of free algebras (cf. also Category).

For a monad $ (T, \eta , \mu ) $ on $ \mathfrak R $, in the Kleisli construction the category $ \mathfrak L $ has as objects the objects of $ \mathfrak R $, and as hom-sets the sets

$$ \mathfrak L (A, B) = \mathfrak R (A, TB). $$

The composition rule for $ \mathfrak L $ assigns to $ f \in \mathfrak L (A, B) $ and $ g \in \mathfrak L (B, C) $ the $ \mathfrak R $- composite:

$$ [A \mathop \rightarrow \limits ^ {T} TB \mathop \rightarrow \limits ^ {T(g)} TTC \mathop \rightarrow \limits ^ {\mu _ {C}} TC ] \in \mathfrak L (A, C); $$

as identity mapping $ 1 _ {A} \in \mathfrak L (A, A) = \mathfrak R (T, TA) $ one uses the $ \mathfrak R $- morphism $ \eta _ {A} : A \mathop \rightarrow \limits TA $.

An adjoint pair $ F: \mathfrak R \mathop \rightarrow \limits \mathfrak L $, $ U: \mathfrak L \mathop \rightarrow \limits \mathfrak R $ is obtained by setting $ F(A)=A $ for $ A \in \mathfrak R $,

$$ F(f) = \eta _ {B} \circ f : A \mathop \rightarrow \limits B \mathop \rightarrow \limits TB \in \mathfrak R (A, TB) = \mathfrak L (A, B) $$

for $ f \in \mathfrak R (A, B) $, $ U(B)=TB $ for $ B \in \mathfrak L $, and $ U(g ) = \mu _ {G} \circ T(g) $ for $ g \in \mathfrak L (B, C)= \mathfrak R (B, TC) $.

Then $ \eta $ will serve as unit for the adjunction, while the co-unit $ \epsilon : FU \mathop \rightarrow \limits {\mathop{\rm Id}\nolimits} _ {\mathfrak L} $ is given by

$$ \epsilon _ {B} = \mathop{\rm Id} _ {T(B)} \in \mathfrak R (TB, TB) = \mathfrak L (FUB, B). $$

Co-algebras are defined in the same manner. In practice, co-algebras very often occur superposed on algebras; a comonad $ G $ will be constructed on a category of algebras of some sort, $ \mathfrak R $, leading to the category $ {} ^ {G} \mathfrak R $ of bi-algebras. An important class of cases involves a monad $ T $ and a cotriple $ G $ on the same category $ \mathfrak R $. There is a standard lifting of $ G $ to a cotriple $ G ^ {*} $ on $ \mathfrak R ^ {T} $. A "TG-bi-algebraTG-bi-algebra" means an object of $ {} ^ {G ^ {*}} ( \mathfrak R ^ {T} ) $; the reverse order is also possible, but rarely occurs, and the objects would not be called bi-algebras.

For the role of comonads in (algebraic) cohomology theories see Cohomology of algebras and [a2], [a3]; particularly [a2] for explicit interpretation.

An adjunction is said to be monadic (or monadable) if the Eilenberg–Moore construction applied to the monad it induces yields an adjunction equivalent to the original one. Many important examples of adjunctions are monadic; for example, for any variety of universal algebras, the forgetful functor from the variety to the category of sets and its left adjoint (the free algebra functor) form a monadic adjunction.

A monad $ (T, \eta , \mu ) $ is said to be idempotent if $ \mu $ is an isomorphism. In this case it can be shown that any $ T $- algebra structure $ \alpha $ on an object $ A $ is necessarily a two-sided inverse for $ \eta _ {A} $, and hence that $ \mathfrak R ^ {T} $ is isomorphic to the full subcategory $ {\mathop{\rm Fix}\nolimits} (T) \subset \mathfrak R $ consisting of all objects $ A $ such that $ \eta _ {A} $ is an isomorphism. $ {\mathop{\rm Fix}\nolimits} (T) $ is a reflective subcategory of $ \mathfrak R $, the left adjoint to the inclusion being given by $ T $ itself. Conversely, for any reflective subcategory of $ \mathfrak R $, the monad on $ \mathfrak R $ induced by the inclusion and its left adjoint is idempotent; thus, the adjunctions corresponding to reflective subcategories are always monadic.

References

[a1]	M. Barr, C. Wells, "Toposes, monads, and theories" , Springer (1985)
[a2]	J.W. Duskin, "$K(\pi,n)$-torsors and the interpretation of "monad" cohomology" Proc. Nat. Acad. Sci. USA , 71 (1974) pp. 2554–2557
[a3]	J.W. Duskin, "Simplicial methods and the interpretation of "monad" cohomology" Mem. Amer. Math. Soc. , 3 (1975)
[a4]	J. Adamek, H. Herrlich, G.E. Strecker, "Abstract and concrete categories" , Wiley (Interscience) (1990)
[a5]	M. Hazewinkel, "Formal groups" , Acad. Press (1978) pp. Sects. 14.5; 14.6, E2
[a6]	H. Appelgate (ed.) et al. (ed.) , Seminar on monads and categorical homology theory ETH 1966/7 , Lect. notes in math. , 80 , Springer (1969)
[a7]	S. Eilenberg, J.C. Moore, "Adjoint functors and monads" Ill. J. Math. , 9 (1965) pp. 381–398
[a8]	S. Eilenberg (ed.) et al. (ed.) , Proc. conf. categorical algebra (La Jolla, 1965) , Springer (1966)

Standard construction

Standard construction is a concept in category theory. Other names are triple, monad and functor-algebra.

Let $\mathfrak{S}$ be a category. A standard construction is a functor $T:\mathfrak{S} \rightarrow \mathfrak{S}$ equipped with natural transformations $\eta:1\rightarrow T$ and $\mu:T^2\rightarrow T$ such that the following diagrams commute: $$ \begin{array}{ccc} T^3 Y & \stackrel{T\mu_Y}{\rightarrow} & T^2 Y \\ \mu_{TY}\downarrow& & \downarrow_Y \\ T^2 & \stackrel{T_y}{\rightarrow} & Y \end{array} $$ $$ \begin{array}{ccccc} TY & \stackrel{TY}{\rightarrow} & T^2Y & \stackrel{T_{\eta Y}}{\leftarrow} & TY \\ & 1\searrow & \downarrow\mu Y & \swarrow1 & \\ & & Y & & \\ \end{array} $$

The basic use of standard constructions in topology is in the construction of various classifying spaces and their algebraic analogues, the so-called bar-constructions.

References

[b1]	J.M. Boardman, R.M. Vogt, "Homotopy invariant algebraic structures on topological spaces" , Springer (1973)
[b2]	J.F. Adams, "Infinite loop spaces" , Princeton Univ. Press (1978)
[b3]	J.P. May, "The geometry of iterated loop spaces" , Lect. notes in math. , 271 , Springer (1972)
[b4]	S. MacLane, "Categories for the working mathematician" , Springer (1971)

Comments

The term "standard construction" was introduced by R. Godement [a1] for want of a better name for this concept. It is now entirely obsolete, having been generally superseded by "monad" (although a minority of authors still use the term "triple" ). Monads have many other uses besides the one mentioned above, for example in the categorical approach to universal algebra (see [a2], [a3]).

References

[c1]	R. Godement, "Théorie des faisceaux" , Hermann (1958)
[c2]	E.G. Manes, "Algebraic theories" , Springer (1976)
[c3]	M. Barr, C. Wells, "Toposes, triples and theories" , Springer (1985)

Ostrowski representation

MSC 11A67

Let $[a_1,a_2,\ldots]$ be the partial quotients of an infinite continued fraction and $(c_n)$ the corresponding continuants, $c_0 = 1$, $c_1 = a_1$ and $c_{n+1} = a_{n+1} c_n + c_{n-1}$. An Ostrowski representation of $N$ is $$ N = \sum_{k=0}^n x_{k+1} c_k $$ where $0 \le x_k \le a_k$ and if $x_k = a_k$ then $x_{k-1} = 0$. Every positive integer $M$ has a unique Ostrowski representation.

When the $a_n$ are all equal to $1$, the $c_n$ are the Fibonacci numbers, and the Ostrowski representation is just the Zeckendorf representation. The addition of two $n$-digit numbers in Ostrowski representation based on a given continued fraction can be computed by three linear passes over the input.

References

• Philipp Hieronymi; Alonza Terry jun. "Ostrowski numeration systems, addition, and finite automata" Notre Dame J. Formal Logic 59 (2018) 215-232 Zbl 06870290

Dirac comb

A sum of Dirac delta-functions supported on a locally finite point set.

References

• Michael Baake, Uwe Gromm; "Aperiodic order", vol.1, Encyclopedia of Mathematics and its Applications 149 (Cambridge, 2013) ISBN 978-0-521-86991-1 Zbl 1295.37001
• Marjorie Senechal; "Quasicrystals and geometry" (Cambridge, 1995) ISBN 0-521-57541-9 Zbl 0828.52007

Delone set

Delaunay set, $(r,R)$-set

A subset $D$ of $\mathbf{R}^n$ which is both discrete: there exists $r>0$ such that the balls of radius $r$ centred on points of $D$ are disjoint; and relatively dense: there exists $R$ such that the balls of radius $R$ centred on points of $D$ cover $\mathbf{R}^n$.

See also Covering and packing.

The spectrum of a Delone set $D$ is defined as the Fourier transform $$ \hat\gamma(s) = \lim_{T\to\infty} \frac{1}{(2T)^n} \sum_{d \in D_T} \exp(-2\pi i\, d\cdot s) $$ where $D_T = D \cap [-T,T]^n$. The spectrum $\hat\gamma$ is a positive measure and has a Lebesgue decomposition into a sum of discrete and continuous measures. If the discrete measure is supported on a countably infinite set $S$, then $D$ is said to satisfy the diffraction condition.

References

• Marjorie Senechal; "Quasicrystals and geometry" (Cambridge, 1995) ISBN 0-521-57541-9 Zbl 0828.52007

SIS model

A simple model in mathematical epidemiology which reduces to the logistic equation. Assume that the population falls into two subgroups, "susceptible" ($S$) and "infected" ($I$), with susceptible members being infected at a rate proportional to the number of infected, and infected members recovering and returning to the susceptible subgroup at a constant rate. We therefore have $$ S' = - \beta S \cdot I + \alpha I $$ and $$ I' = \beta S \cdot I - \alpha I $$ Since $S+I = N$ is constant, we have $I' = r I (1- k^{-1} I)$ where $r = \beta N - \alpha$ is the growth rate and $k = r / \beta$. The basic reproduction number $R_0 = \beta N / \alpha$. If $R_0 < 1$, so that $r<0$, then $I$ decreases to zero. Otherwise we have the explicit solution $$ I(t) = \frac{ k B e^{rt} }{ 1 + B e^{rt} } $$ where $B= I(0) / (k - I(0))$ and $I(t)$ tends to $k$ as $t \to \infty$

Reference

• Maia Martcheva, "An Introduction to Mathematical Epdidemiology" Texts in Applied Mathematics 41 (Springer, 2015) ISBN 978-1-4899-7611-6 Zbl 1333.92006

Pregroup

A pregroup generalises the notion of a free group with amalgamation. A pregroup is a partially ordered monoid $(M,{\cdot},1,{\ge})$ with left and right adjoint maps $L$ and $R$ satisfying $$ x^L \cdot x \ge 1 \ge x \cdot x^L $$ $$ x \cdot x^R \ge 1 \ge x^R \cdot x $$ It follows that $x^{LR} = x = x^{RL}$

A partially ordered group is a pregroup, with both adjoint maps being group inversion.

References

• J. Stallings, "Group theory and three-dimensional manifolds" Yale Univ. Monogr. 4 (1971) Zbl 0241.57001

Freiman homomorphism

A map $\phi$ defined on a subset $A$ of an additive group $G$ to a group $H$ such that for $a_1,a_2,a_3,a-4 \in A$ $$ a_1 + a_2 = a_3 + a_4 \ \Rightarrow\ \phi(a_1) + \phi(a_2) = \phi(a_3) + \phi(a_4) $$

Clearly an affine map $x \mapsto \psi(x) + b$ with $\psi$ a group homomorphism and $b \in H$ is a Frieman homomorphism. A notable problem in additive combinatorics is to find conditions on $A$ that require every Freiman homomorphism to be affine.

More generally, a Freiman homomorphism of order $k$ satisfies the corresponding property for $k$-tuples with equal sums.

References

• Melvyn B. Nathanson, "Additive Number Theory: Inverse Problems and the Geometry of Sumsets", Graduate Texts in Mathematics 165 (Springer, 1996) ISBN 0-387-94655-1
• Terence Tao, Van H. Vu; "Additive Combinatorics", Cambridge Studies in Advanced Mathematics 105 (Cambridge University Press, 2006)ISBN 1-1394-5834-5
• David J. Grynkiewicz, "Structural Additive Theory", Developments in Mathematics 30 (Springer, 2013) ISBN 3-319-00416-6