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Assume $p$ is prime and $p\ge 3$.

Through experimentation, I can see that it's probably true. Using Wilson's theorem and Fermat's little theorem, it's equivalent to saying $2^2 4^2 6^2 \cdots (p-1)^2 = (-1)^{\frac{p+1}{2}}$ mod $p$, but I can't figure out any more than that.

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    @KCd: Thanks! That was exactly what I needed. I'll mark it as accepted if you put that in an answer instead of a comment.2012-04-13

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$(p-1)!=1\cdot2\cdots\frac{p-1}{2}\cdot\frac{p+1}{2}\cdots(p-2)(p-1)$ We have the congruences $\begin{align*} p-1&\equiv -1\pmod p\\ p-2&\equiv -2\pmod p\\ &\vdots\\ \frac{p+1}{2}&\equiv -\frac{p-1}{2}\pmod{p}\end{align*}$ Rearranging the factors produces

$(p-1)!\equiv 1\cdot(-1)\cdot2\cdot(-2)\cdots\frac{p-1}{2}\cdot\left(-\frac{p-1}{2}\right)\pmod p.$ $\therefore (p-1)!\equiv (-1)^{\frac{p-1}{2}}\left(1\cdot2\cdots\frac{p-1}{2}\right)^{2}\pmod p.$ From Wilson's theorem, $(p-1)!\equiv -1\pmod{p}$ Thus $-1\equiv (-1)^{\frac{p-1}{2}}\left[\left(\frac{p-1}{2}\right)!\right]^{2}\pmod{p}$ It follows that $\left[\left(\frac{p-1}{2}\right)!\right]^{2}\equiv (-1)^{\frac{p+1}{2}}\pmod{p}$.

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    @ArturoMagidin: Thanks arturo for the $\LaTeX$ tip2012-04-16