Content-Length: 123824 | pFad | http://en.m.wikipedia.org/wiki/Schur_functor

Schur functor - Wikipedia

In mathematics, especially in the field of representation theory, Schur functors (named after Issai Schur) are certain functors from the category of modules over a fixed commutative ring to itself. They generalize the constructions of exterior powers and symmetric powers of a vector space. Schur functors are indexed by Young diagrams in such a way that the horizontal diagram with n cells corresponds to the nth symmetric power functor, and the vertical diagram with n cells corresponds to the nth exterior power functor. If a vector space V is a representation of a group G, then also has a natural action of G for any Schur functor .

Definition

edit

Schur functors are indexed by partitions and are described as follows. Let R be a commutative ring, E an R-module and λ a partition of a positive integer n. Let T be a Young tableau of shape λ, thus indexing the factors of the n-fold direct product, E × E × ... × E, with the boxes of T. Consider those maps of R-modules   satisfying the following conditions

  1.   is multilinear,
  2.   is alternating in the entries indexed by each column of T,
  3.   satisfies an exchange condition stating that if   are numbers from column i of T then
 

where the sum is over n-tuples x′ obtained from x by exchanging the elements indexed by I with any   elements indexed by the numbers in column   (in order).

The universal R-module   that extends   to a mapping of R-modules   is the image of E under the Schur functor indexed by λ.

For an example of the condition (3) placed on   suppose that λ is the partition   and the tableau T is numbered such that its entries are 1, 2, 3, 4, 5 when read top-to-bottom (left-to-right). Taking   (i.e., the numbers in the second column of T) we have

 

while if   then

 

Examples

edit

Fix a vector space V over a field of characteristic zero. We identify partitions and the corresponding Young diagrams. The following descriptions hold:[1]

  • For a partition λ = (n) the Schur functor Sλ(V) = Symn(V).
  • For a partition λ = (1, ..., 1) (repeated n times) the Schur functor Sλ(V) = Λn(V).
  • For a partition λ = (2, 1) the Schur functor Sλ(V) is the cokernel of the comultiplication map of exterior powers Λ3(V) → Λ2(V) ⊗ V.
  • For a partition λ = (2, 2) the Schur functor Sλ(V) is the quotient of Λ2(V) ⊗ Λ2(V) by the images of two maps. One is the composition Λ3(V) ⊗ V → Λ2(V) ⊗ VV → Λ2(V) ⊗ Λ2(V), where the first map is the comultiplication along the first coordinate. The other map is a comultiplication Λ4(V) → Λ2(V) ⊗ Λ2(V).
  • For a partition λ = (n, 1, ..., 1), with 1 repeated m times, the Schur functor Sλ(V) is the quotient of Λn(V) ⊗ Symm(V) by the image of the composition of the comultiplication in exterior powers and the multiplication in symmetric powers:
     

Applications

edit

Let V be a complex vector space of dimension k. It's a tautological representation of its automorphism group GL(V). If λ is a diagram where each row has no more than k cells, then Sλ(V) is an irreducible GL(V)-representation of highest weight λ. In fact, any rational representation of GL(V) is isomorphic to a direct sum of representations of the form Sλ(V) ⊗ det(V)m, where λ is a Young diagram with each row strictly shorter than k, and m is any (possibly negative) integer.

In this context Schur-Weyl duality states that as a GL(V)-module

 

where   is the number of standard young tableaux of shape λ. More generally, we have the decomposition of the tensor product as  -bimodule

 

where   is the Specht module indexed by λ. Schur functors can also be used to describe the coordinate ring of certain flag varieties.

Plethysm

edit

For two Young diagrams λ and μ consider the composition of the corresponding Schur functors Sλ(Sμ(−)). This composition is called a plethysm of λ and μ. From the general theory it is known[1] that, at least for vector spaces over a characteristic zero field, the plethysm is isomorphic to a direct sum of Schur functors. The problem of determining which Young diagrams occur in that description and how to calculate their multiplicities is open, aside from some special cases like Symm(Sym2(V)).

See also

edit

References

edit
  1. ^ a b Weyman, Jerzy (2003). Cohomology of Vector Bundles and Syzygies. Cambridge University Press. doi:10.1017/CBO9780511546556. ISBN 9780511546556.
  • J. Towber, Two new functors from modules to algebras, J. Algebra 47 (1977), 80-104. doi:10.1016/0021-8693(77)90211-3
  • W. Fulton, Young Tableaux, with Applications to Representation Theory and Geometry. Cambridge University Press, 1997, ISBN 0-521-56724-6
edit








ApplySandwichStrip

pFad - (p)hone/(F)rame/(a)nonymizer/(d)eclutterfier!      Saves Data!


--- a PPN by Garber Painting Akron. With Image Size Reduction included!

Fetched URL: http://en.m.wikipedia.org/wiki/Schur_functor

Alternative Proxies:

Alternative Proxy

pFad Proxy

pFad v3 Proxy

pFad v4 Proxy