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 Post subject: binomial coefficient
PostPosted: Mon, 11 Dec 2017 15:20:43 UTC 
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(1) Let p be a prime. What is \displaystyle\binom{2p}{p} modulo p^2?

(2) Let p be an odd prime. What is \displaystyle\binom{3p}{p} modulo p^3?

Hint:
Spoiler:
For (1), recall \displaystyle\binom{a+b}{r}=\sum_{j=0}^r\binom{a}{j}\binom{b}{r-j} and \displaystyle\binom{p}{r}\equiv 0\pmod{p} except for r=0,p. For (2), you need to sharpen this a bit.


Big hint which trivializes the problem:
Spoiler:
Wolstenholme's theorem.

_________________
\begin{aligned}
Spin(1)&=O(1)=\mathbb{Z}/2&\quad&\text{and}\\
Spin(2)&=U(1)=SO(2)&&\text{are obvious}\\
Spin(3)&=Sp(1)=SU(2)&&\text{by }q\mapsto(\mathop{\mathrm{Im}}\mathbb{H}\ni p\mapsto qp\bar{q})\\
Spin(4)&=Sp(1)\times Sp(1)&&\text{by }(q_1,q_2)\mapsto(\mathbb{H}\ni p\mapsto q_1p\bar{q_2})\\
Spin(5)&=Sp(2)&&\text{by }\mathbb{HP}^1\cong S^4_{round}\hookrightarrow\mathbb{R}^5\\
Spin(6)&=SU(4)&&\text{by the irrep }\Lambda_+\mathbb{C}^4
\end{aligned}


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