Pochhammer 記號
Pochhammer symbol$ (x)^n,$ (x)_n
昇冪$ (x)^0:=1,$ (x)^n:=\prod^{n-1}_{j=0}(x+j)
$ \frac{\Gamma(x+n)}{\Gamma(x)},$ \frac{(x+n-1)!}{(x-1)!}
降冪$ (x)_n:=\prod^{n-1}_{j}(x-j)
$ \frac{\Gamma(x+1)}{\Gamma(x-n+1)},$ \frac{x!}{(x-n)!},(x\ge n)
階乘は$ n!=(1)^n=(n)_n