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5 votes
Accepted

Symplectic form on the third symmetric power of a plane

There's nothing miraculous about this: Here is an explicit formula: Let $x$ and $y$ be a basis for $V$. Then $A,B\in S^3(V)$ can be written in the form $$ A = a_{-3}\,x^3+3a_{-1}\,x^2y+3a_1\,xy^2+a …
Robert Bryant's user avatar
27 votes
Accepted

Alternate and symmetric matrices

I feel that framing this question in terms of matrices rather than bilinear forms on a vector space obscures what is actually going on and makes it harder to understand what needs to be proved. Here …
Robert Bryant's user avatar
6 votes
Accepted

Symplectic block-diagonalization of a complex symmetric matrix

This fails even for $n=1$. In this case, the matrix $$ A = \begin{pmatrix} 1&0\\0&0\end{pmatrix} $$ can't be diagonalized in the form that you want because it's not zero, yet its determinant vanishes …
Robert Bryant's user avatar