Tuesday, 13 September 2011

invariant theory - Is Symn(V)congSymn(V)ast naturally in positive characteristic?

Background/motivation



It is a classical fact that we have a natural isomorphism Symn(V)congSymn(V)ast for vector spaces V over a field k of characteristic 0. One way to see this is the following.



On the one hand elements of Symn(V) are symmetric powers of degree n of linear forms on V, so they can be identified with homogeneous polynomials of degree n on V. On the other hand elements of Symn(V)ast are linear functionals on SymnV; by the universal property of SymnV these correspond to n-multilinear symmetric forms on V. The isomorphism is then as follows.



An n-multilinear symmetric form phi corresponds to the homogeneous polynomial p(v)=phi(v,dots,v). In the other direction to a polynomial p(v) we attach the multinear form obtained by polarization phi(v1,dots,vn)=frac1n!sumIsubset[n](1)nsharpIp(sumiinIvi). Here [n] is the set lbrace1,dots,nrbrace.



Problem



Of course this will not work for n greater than the characteristic of k if the latter is positive.



One can expect that an isomorphism Symn(V)congSymn(V)ast holds also in positive characteristic, and that this should be trivially true by using the universal properties of the symmetric powers. The problem is that if I try to define a natural map between the two spaces using the universal properties I have at some point to divide by n! anyway.



Still there may be some natural isomorphism that I cannot see. Or maybe there is not a natural isomorphism, but I don't know how to prove this.




Is there a natural isomoprhism Symn(V)congSymn(V)ast in positive characteristic?


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