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For example, over fields with characteristic 2, there exist nonzero symmetric nilpotent matrices, and nonzero matrices could be simultaneously symmetric and anti-symmetric. I wonder why characteristic 2 makes such fields so special.

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You could say that $2$ is an odd prime in that respect. :-) – lhf Dec 4 '12 at 2:23
up vote 12 down vote accepted

All fields of nonzero characteristic are 'pathological' in some sense. It's just easier to trip over a problem with $2$ than a problem with, say, $1319$.

Symmetric nilpotents exist in all characteristics. For example, in characteristic 3, you have

$$ \left( \begin{matrix}1 & 1 & 1 \\ 1 & 1 & 1 \\ 1 & 1 & 1 \end{matrix} \right) $$

as an example of a matrix that squares to zero. It's easy to generalize this to any positive characteristic.

Squaring behaves strangely in characteristic 2. Among the oddities is that there is only one square root of 1. In some sense, this is responsible for the thing with symmetric and anti-symmetric.

In characteristic 3, it's cubing that's strange, and so forth.

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Mostly, it's because $a=-a$ in fields of characteristic $2$.

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«Mostly»! ${}{}{}{}{}$ – Mariano Suárez-Alvarez Dec 4 '12 at 3:07

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