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''(real)''
 
''(real)''
  
The manifold <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877701.png" /> of orthonormal <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877702.png" />-frames in an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877703.png" />-dimensional Euclidean space. In a similar way one defines a complex Stiefel manifold <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877704.png" /> and a quaternion Stiefel manifold <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877705.png" />. Stiefel manifolds are compact real-analytic manifolds, and also homogeneous spaces of the classical compact groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877706.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877707.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877708.png" />, respectively. In particular, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s0877709.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777010.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777011.png" /> are the spheres, the Stiefel manifold <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777012.png" /> is the manifold of unit vectors tangent to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777013.png" />, the Stiefel manifolds <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777014.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777015.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777016.png" /> are identified with the groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777017.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777018.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777019.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777020.png" /> — with the group <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777021.png" />. Sometimes non-compact Stiefel manifolds, consisting of all possible <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777022.png" />-frames in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777023.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777024.png" /> or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777025.png" />, are considered.
+
The manifold $  V _ {n,k} $
 +
of orthonormal $  k $-
 +
frames in an $  n $-
 +
dimensional Euclidean space. In a similar way one defines a complex Stiefel manifold $  W _ {n,k} $
 +
and a quaternion Stiefel manifold $  X _ {n,k} $.  
 +
Stiefel manifolds are compact real-analytic manifolds, and also homogeneous spaces of the classical compact groups $  O( n) $,  
 +
$  U( n) $
 +
and $  \mathop{\rm Sp} ( n) $,  
 +
respectively. In particular, $  V _ {n,1} = S  ^ {n-} 1 $,
 +
$  W _ {n,1} = S  ^ {2n-} 1 $,  
 +
$  X _ {n,1} = S  ^ {4n-} 1 $
 +
are the spheres, the Stiefel manifold $  V _ {n,2} $
 +
is the manifold of unit vectors tangent to $  S  ^ {n-} 1 $,  
 +
the Stiefel manifolds $  V _ {n,n} $,
 +
$  W _ {n,n} $,  
 +
$  X _ {n,n} $
 +
are identified with the groups $  O( n) $,  
 +
$  U( n) $,  
 +
$  \mathop{\rm Sp} ( n) $,  
 +
and $  V _ {n,n-} 1 $—  
 +
with the group $  \mathop{\rm SO} ( n) $.  
 +
Sometimes non-compact Stiefel manifolds, consisting of all possible $  k $-
 +
frames in $  \mathbf R  ^ {n} $,  
 +
$  \mathbf C  ^ {n} $
 +
or $  \mathbf H  ^ {n} $,  
 +
are considered.
  
 
These manifolds were introduced by E. Stiefel [[#References|[1]]] in connection with systems of linearly independent vector fields on smooth manifolds. First started in [[#References|[1]]], studies on the topology of Stiefel manifolds led later to the complete calculation of their cohomology rings (see [[#References|[2]]], [[#References|[3]]]). In particular,
 
These manifolds were introduced by E. Stiefel [[#References|[1]]] in connection with systems of linearly independent vector fields on smooth manifolds. First started in [[#References|[1]]], studies on the topology of Stiefel manifolds led later to the complete calculation of their cohomology rings (see [[#References|[2]]], [[#References|[3]]]). In particular,
  
<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/s087/s087770/s08777026.png" /></td> </tr></table>
+
$$
 +
H  ^  \star  ( W _ {n,k} , \mathbf Z )  = \Lambda _ {\mathbf Z} ( x _ {2n-} 1 , x _ {2n-} 3 \dots x _ {2(} n- k)+ 1 ),
 +
$$
 +
 
 +
$$
 +
H  ^  \star  ( X _ {n,k} , \mathbf Z )  = \Lambda _ {\mathbf Z} ( x _ {4n-} 1 , x _ {4n-} 5 \dots x _ {4(} n- k)+ 3 ),
 +
$$
 +
 
 +
$  H  ^  \star  ( V _ {n,k} , \mathbf Z _ {2} ) $
 +
is a commutative algebra with generators  $  x _ {n-} k \dots x _ {n-} 1 $
 +
and relations
 +
 
 +
$$
 +
x _ {i} x _ {j}  = \left \{
 +
 
 +
\begin{array}{lll}
 +
x _ {i+} j  & \textrm{ for }  &i+ j \leq  n- 1,  \\
 +
0 & \textrm{ for }  &i+ j > n- 1  \\
 +
\end{array}
  
<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/s087/s087770/s08777027.png" /></td> </tr></table>
+
\right .$$
  
<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777028.png" /> is a commutative algebra with generators <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777029.png" /> and relations
+
(everywhere above,  $  x _ {l} $
 +
denotes an element of order  $  l $).  
 +
Real, complex and quaternion Stiefel manifolds are aspherical in dimensions not exceeding  $  n- k- 1 $,
 +
$  2( n- k) $
 +
and  $  4( n- k)+ 2 $,
 +
respectively. Moreover,
  
<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/s087/s087770/s08777030.png" /></td> </tr></table>
+
$$
 +
\pi _ {n-} k ( V _ {n,k} )  \cong  \left \{
  
(everywhere above, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777031.png" /> denotes an element of order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777032.png" />). Real, complex and quaternion Stiefel manifolds are aspherical in dimensions not exceeding <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777033.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777034.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777035.png" />, respectively. Moreover,
+
\begin{array}{ll}
 +
\mathbf Z  & \textrm{ if }  k= 1  \textrm{ or }  n- k  \textrm{ is  even  }, \\
 +
\mathbf Z _ {2}  & \textrm{ if }  k> 1  \textrm{ or }  n- k  \textrm{ is  odd  } ;  \\
 +
\end{array}
  
<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/s087/s087770/s08777036.png" /></td> </tr></table>
+
\right .$$
  
<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/s087/s087770/s08777037.png" /></td> </tr></table>
+
$$
 +
\pi _ {2(} n- k)+ 1 ( W _ {n,k} )  \cong  \pi _ {4(} n- k)+ 3 ( X _ {n,k} )  \cong  \mathbf Z .
 +
$$
  
 
The computation of other homotopy groups of Stiefel manifolds is discussed in [[#References|[5]]].
 
The computation of other homotopy groups of Stiefel manifolds is discussed in [[#References|[5]]].
Line 23: Line 89:
 
====References====
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  E. Stiefel,  "Richtungsfelder und Fernparallelismus in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777038.png" />-dimensionalen Mannigfaltigkeiten"  ''Comm. Math. Helv.'' , '''8''' :  4  (1935–1936)  pp. 305–353</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  A. Borel,  , ''Fibre spaces and their applications'' , Moscow  (1958)  pp. 163–246  (In Russian; translated from French)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  N.E. Steenrod,  D.B.A. Epstein,  "Cohomology operations" , Princeton Univ. Press  (1962)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  V.A. Rokhlin,  D.B. Fuks,  "Beginner's course in topology. Geometric chapters" , Springer  (1984)  (Translated from Russian)</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  ''Itogi Nauk. Algebra. Topol. Geom.''  (1971)  pp. 71–122</TD></TR></table>
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  E. Stiefel,  "Richtungsfelder und Fernparallelismus in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777038.png" />-dimensionalen Mannigfaltigkeiten"  ''Comm. Math. Helv.'' , '''8''' :  4  (1935–1936)  pp. 305–353</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  A. Borel,  , ''Fibre spaces and their applications'' , Moscow  (1958)  pp. 163–246  (In Russian; translated from French)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  N.E. Steenrod,  D.B.A. Epstein,  "Cohomology operations" , Princeton Univ. Press  (1962)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  V.A. Rokhlin,  D.B. Fuks,  "Beginner's course in topology. Geometric chapters" , Springer  (1984)  (Translated from Russian)</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  ''Itogi Nauk. Algebra. Topol. Geom.''  (1971)  pp. 71–122</TD></TR></table>
 
 
  
 
====Comments====
 
====Comments====
 
For homotopy groups of Stiefel manifolds see also .
 
For homotopy groups of Stiefel manifolds see also .
  
Another (and better) frequently used notation for the Stiefel manifolds <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777039.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777040.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777041.png" /> is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777042.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777043.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777044.png" />, generalizing to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777045.png" /> where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777046.png" /> is an appropriate vector space.
+
Another (and better) frequently used notation for the Stiefel manifolds $  V _ {n,k }  $,
 +
$  W _ {n,k }  $
 +
and $  X _ {n,k }  $
 +
is $  V _ {k} ( \mathbf R  ^ {n} ) $,  
 +
$  V _ {k} ( \mathbf C  ^ {n} ) $,  
 +
$  V _ {k} ( \mathbf H  ^ {n} ) $,  
 +
generalizing to $  V _ {k} ( E ) $
 +
where $  E $
 +
is an appropriate vector space.
  
 
As homogeneous spaces these Stiefel manifolds are equal to, respectively,
 
As homogeneous spaces these Stiefel manifolds are equal to, respectively,
  
<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/s087/s087770/s08777047.png" /></td> </tr></table>
+
$$
 +
V_{k} ( \mathbf R^n) = \frac{O(n)}{O(n- k)} = \frac{ \mathop{\rm SO} (n)}{\mathop{\rm SO} (n-k)},
 +
$$
  
<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/s087/s087770/s08777048.png" /></td> </tr></table>
+
$$
 +
V_{k} ( \mathbf C ^{n}) = \frac{U(n)}{U(n- k)} = \frac{ \mathop{\rm SU} (n)}{ \mathop{\rm SU} (n-k)},
 +
$$
  
<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/s087/s087770/s08777049.png" /></td> </tr></table>
+
$$
 +
V _ {k} ( \mathbf H  ^ {n} )  =
 +
\frac{ \mathop{\rm Sp} ( n) }{ \mathop{\rm Sp} ( n- k) }.
 +
$$
  
The natural quotient mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777050.png" />, etc., assigns to an orthogonal, etc., matrix the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777051.png" />-frame consisting of its first <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777052.png" /> columns.
+
The natural quotient mapping $  O( n) \rightarrow V _ {k} ( \mathbf R  ^ {n} ) $,  
 +
etc., assigns to an orthogonal, etc., matrix the $  k $-
 +
frame consisting of its first $  k $
 +
columns.
  
There are canonical mappings from the Stiefel manifolds to the Grassmann manifolds (cf. [[Grassmann manifold|Grassmann manifold]]):
+
There are canonical mappings from the Stiefel manifolds to the Grassmann manifolds (cf. [[Grassmann manifold]]):
  
<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/s087/s087770/s08777053.png" /></td> </tr></table>
+
$$
 +
V _ {k} ( E)  \rightarrow  \mathop{\rm Gr}_{k} ( E) ,
 +
$$
  
which assign to a <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777054.png" />-frame the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777055.png" />-dimensional subspace spanned by that frame. This exhibits the Grassmann manifolds as homogeneous spaces:
+
which assign to a $  k $-
 +
frame the $  k $-
 +
dimensional subspace spanned by that frame. This exhibits the Grassmann manifolds as homogeneous spaces:
  
<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/s087/s087770/s08777056.png" /></td> </tr></table>
+
$$
 +
\mathop{\rm Gr}_{k} (\mathbf R^{n} )  = \frac {O(n)}{O(k)\times O(n-k)} ,
 +
$$
  
 
etc.
 
etc.
  
Given an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777057.png" />-dimensional (real, complex, quaternionic) [[Vector bundle|vector bundle]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777058.png" /> over a space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777059.png" />, the associated Stiefel bundles <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777060.png" /> have the fibres <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777061.png" /> over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777062.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777063.png" /> is the fibre of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777064.png" /> over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777065.png" />. Similarly one has the Grassmann bundle <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777066.png" />, whose fibre over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777067.png" /> is the Grassmann manifold <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777068.png" />.
+
Given an $  n $-
 +
dimensional (real, complex, quaternionic) [[vector bundle]] $  E $
 +
over a space $  X $,  
 +
the associated Stiefel bundles $  V _ {k} ( E) $
 +
have the fibres $  V _ {k} ( E _ {x} ) $
 +
over $  x \in X $,  
 +
where $  E _ {x} $
 +
is the fibre of $  E $
 +
over $  x $.  
 +
Similarly one has the Grassmann bundle $  \mathop{\rm Gr} _ {k} ( E) $,  
 +
whose fibre over $  x \in X $
 +
is the Grassmann manifold $  \mathop{\rm Gr} _ {k} ( E _ {x} ) $.
  
 
====References====
 
====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  D. Husemoller,  "Fibre bundles" , McGraw-Hill  (1966)</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  J. Dieudonné,  "A history of algebraic and differential topology 1900–1960" , Birkhäuser  (1989)</TD></TR><TR><TD valign="top">[a3a]</TD> <TD valign="top">  G.F. Paechter,  "The groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777069.png" />"  ''Quarterly J. Math.'' , '''7'''  (1956)  pp. 249–268</TD></TR><TR><TD valign="top">[a3b]</TD> <TD valign="top">  G.F. Paechter,  "The groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777070.png" />"  ''Quarterly J. Math.'' , '''9'''  (1958)  pp. 8–27</TD></TR><TR><TD valign="top">[a3c]</TD> <TD valign="top">  G.F. Paechter,  "The groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777071.png" />"  ''Quarterly J. Math.'' , '''10'''  (1959)  pp. 17–37; 241–260</TD></TR><TR><TD valign="top">[a3d]</TD> <TD valign="top">  G.F. Paechter,  "The groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087770/s08777072.png" />"  ''Quarterly J. Math.'' , '''11'''  (1960)  pp. 1–16</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  M.W. Hirsch,  "Differential topology" , Springer  (1976)  pp. 4, 78</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  J.W. Milnor,  J.D. Stasheff,  "Characteristic classes" , Princeton Univ. Press  (1974)</TD></TR></table>
+
<table>
 +
<TR><TD valign="top">[a1]</TD> <TD valign="top">  D. Husemoller,  "Fibre bundles" , McGraw-Hill  (1966)</TD></TR>
 +
<TR><TD valign="top">[a2]</TD> <TD valign="top">  J. Dieudonné,  "A history of algebraic and differential topology 1900–1960" , Birkhäuser  (1989)</TD></TR>
 +
<TR><TD valign="top">[a3a]</TD> <TD valign="top">  G.F. Paechter,  "The groups $\pi_r(V_{n,m})$"  ''Quarterly J. Math.'' , '''7'''  (1956)  pp. 249–268</TD></TR>
 +
<TR><TD valign="top">[a3b]</TD> <TD valign="top">  G.F. Paechter,  "The groups $\pi_r(V_{n,m})$"  ''Quarterly J. Math.'' , '''9'''  (1958)  pp. 8–27</TD></TR>
 +
<TR><TD valign="top">[a3c]</TD> <TD valign="top">  G.F. Paechter,  "The groups $\pi_r(V_{n,m})$"  ''Quarterly J. Math.'' , '''10'''  (1959)  pp. 17–37; 241–260</TD></TR>
 +
<TR><TD valign="top">[a3d]</TD> <TD valign="top">  G.F. Paechter,  "The groups $\pi_r(V_{n,m})$"  ''Quarterly J. Math.'' , '''11'''  (1960)  pp. 1–16</TD></TR>
 +
<TR><TD valign="top">[a4]</TD> <TD valign="top">  M.W. Hirsch,  "Differential topology" , Springer  (1976)  pp. 4, 78</TD></TR>
 +
<TR><TD valign="top">[a5]</TD> <TD valign="top">  J.W. Milnor,  J.D. Stasheff,  "Characteristic classes" , Princeton Univ. Press  (1974)</TD></TR>
 +
</table>

Latest revision as of 19:44, 7 June 2024


(real)

The manifold $ V _ {n,k} $ of orthonormal $ k $- frames in an $ n $- dimensional Euclidean space. In a similar way one defines a complex Stiefel manifold $ W _ {n,k} $ and a quaternion Stiefel manifold $ X _ {n,k} $. Stiefel manifolds are compact real-analytic manifolds, and also homogeneous spaces of the classical compact groups $ O( n) $, $ U( n) $ and $ \mathop{\rm Sp} ( n) $, respectively. In particular, $ V _ {n,1} = S ^ {n-} 1 $, $ W _ {n,1} = S ^ {2n-} 1 $, $ X _ {n,1} = S ^ {4n-} 1 $ are the spheres, the Stiefel manifold $ V _ {n,2} $ is the manifold of unit vectors tangent to $ S ^ {n-} 1 $, the Stiefel manifolds $ V _ {n,n} $, $ W _ {n,n} $, $ X _ {n,n} $ are identified with the groups $ O( n) $, $ U( n) $, $ \mathop{\rm Sp} ( n) $, and $ V _ {n,n-} 1 $— with the group $ \mathop{\rm SO} ( n) $. Sometimes non-compact Stiefel manifolds, consisting of all possible $ k $- frames in $ \mathbf R ^ {n} $, $ \mathbf C ^ {n} $ or $ \mathbf H ^ {n} $, are considered.

These manifolds were introduced by E. Stiefel [1] in connection with systems of linearly independent vector fields on smooth manifolds. First started in [1], studies on the topology of Stiefel manifolds led later to the complete calculation of their cohomology rings (see [2], [3]). In particular,

$$ H ^ \star ( W _ {n,k} , \mathbf Z ) = \Lambda _ {\mathbf Z} ( x _ {2n-} 1 , x _ {2n-} 3 \dots x _ {2(} n- k)+ 1 ), $$

$$ H ^ \star ( X _ {n,k} , \mathbf Z ) = \Lambda _ {\mathbf Z} ( x _ {4n-} 1 , x _ {4n-} 5 \dots x _ {4(} n- k)+ 3 ), $$

$ H ^ \star ( V _ {n,k} , \mathbf Z _ {2} ) $ is a commutative algebra with generators $ x _ {n-} k \dots x _ {n-} 1 $ and relations

$$ x _ {i} x _ {j} = \left \{ \begin{array}{lll} x _ {i+} j & \textrm{ for } &i+ j \leq n- 1, \\ 0 & \textrm{ for } &i+ j > n- 1 \\ \end{array} \right .$$

(everywhere above, $ x _ {l} $ denotes an element of order $ l $). Real, complex and quaternion Stiefel manifolds are aspherical in dimensions not exceeding $ n- k- 1 $, $ 2( n- k) $ and $ 4( n- k)+ 2 $, respectively. Moreover,

$$ \pi _ {n-} k ( V _ {n,k} ) \cong \left \{ \begin{array}{ll} \mathbf Z & \textrm{ if } k= 1 \textrm{ or } n- k \textrm{ is even }, \\ \mathbf Z _ {2} & \textrm{ if } k> 1 \textrm{ or } n- k \textrm{ is odd } ; \\ \end{array} \right .$$

$$ \pi _ {2(} n- k)+ 1 ( W _ {n,k} ) \cong \pi _ {4(} n- k)+ 3 ( X _ {n,k} ) \cong \mathbf Z . $$

The computation of other homotopy groups of Stiefel manifolds is discussed in [5].

References

[1] E. Stiefel, "Richtungsfelder und Fernparallelismus in -dimensionalen Mannigfaltigkeiten" Comm. Math. Helv. , 8 : 4 (1935–1936) pp. 305–353
[2] A. Borel, , Fibre spaces and their applications , Moscow (1958) pp. 163–246 (In Russian; translated from French)
[3] N.E. Steenrod, D.B.A. Epstein, "Cohomology operations" , Princeton Univ. Press (1962)
[4] V.A. Rokhlin, D.B. Fuks, "Beginner's course in topology. Geometric chapters" , Springer (1984) (Translated from Russian)
[5] Itogi Nauk. Algebra. Topol. Geom. (1971) pp. 71–122

Comments

For homotopy groups of Stiefel manifolds see also .

Another (and better) frequently used notation for the Stiefel manifolds $ V _ {n,k } $, $ W _ {n,k } $ and $ X _ {n,k } $ is $ V _ {k} ( \mathbf R ^ {n} ) $, $ V _ {k} ( \mathbf C ^ {n} ) $, $ V _ {k} ( \mathbf H ^ {n} ) $, generalizing to $ V _ {k} ( E ) $ where $ E $ is an appropriate vector space.

As homogeneous spaces these Stiefel manifolds are equal to, respectively,

$$ V_{k} ( \mathbf R^n) = \frac{O(n)}{O(n- k)} = \frac{ \mathop{\rm SO} (n)}{\mathop{\rm SO} (n-k)}, $$

$$ V_{k} ( \mathbf C ^{n}) = \frac{U(n)}{U(n- k)} = \frac{ \mathop{\rm SU} (n)}{ \mathop{\rm SU} (n-k)}, $$

$$ V _ {k} ( \mathbf H ^ {n} ) = \frac{ \mathop{\rm Sp} ( n) }{ \mathop{\rm Sp} ( n- k) }. $$

The natural quotient mapping $ O( n) \rightarrow V _ {k} ( \mathbf R ^ {n} ) $, etc., assigns to an orthogonal, etc., matrix the $ k $- frame consisting of its first $ k $ columns.

There are canonical mappings from the Stiefel manifolds to the Grassmann manifolds (cf. Grassmann manifold):

$$ V _ {k} ( E) \rightarrow \mathop{\rm Gr}_{k} ( E) , $$

which assign to a $ k $- frame the $ k $- dimensional subspace spanned by that frame. This exhibits the Grassmann manifolds as homogeneous spaces:

$$ \mathop{\rm Gr}_{k} (\mathbf R^{n} ) = \frac {O(n)}{O(k)\times O(n-k)} , $$

etc.

Given an $ n $- dimensional (real, complex, quaternionic) vector bundle $ E $ over a space $ X $, the associated Stiefel bundles $ V _ {k} ( E) $ have the fibres $ V _ {k} ( E _ {x} ) $ over $ x \in X $, where $ E _ {x} $ is the fibre of $ E $ over $ x $. Similarly one has the Grassmann bundle $ \mathop{\rm Gr} _ {k} ( E) $, whose fibre over $ x \in X $ is the Grassmann manifold $ \mathop{\rm Gr} _ {k} ( E _ {x} ) $.

References

[a1] D. Husemoller, "Fibre bundles" , McGraw-Hill (1966)
[a2] J. Dieudonné, "A history of algebraic and differential topology 1900–1960" , Birkhäuser (1989)
[a3a] G.F. Paechter, "The groups $\pi_r(V_{n,m})$" Quarterly J. Math. , 7 (1956) pp. 249–268
[a3b] G.F. Paechter, "The groups $\pi_r(V_{n,m})$" Quarterly J. Math. , 9 (1958) pp. 8–27
[a3c] G.F. Paechter, "The groups $\pi_r(V_{n,m})$" Quarterly J. Math. , 10 (1959) pp. 17–37; 241–260
[a3d] G.F. Paechter, "The groups $\pi_r(V_{n,m})$" Quarterly J. Math. , 11 (1960) pp. 1–16
[a4] M.W. Hirsch, "Differential topology" , Springer (1976) pp. 4, 78
[a5] J.W. Milnor, J.D. Stasheff, "Characteristic classes" , Princeton Univ. Press (1974)
How to Cite This Entry:
Stiefel manifold. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Stiefel_manifold&oldid=49446
This article was adapted from an original article by A.L. Onishchik (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article