# double factorial x!!, e.g. 4!!; #doublefactorial(n_):= If(n > 1, n*doublefactorial(n-2), If(n==0 or n==1, 1)); #doublefactorial(n_):= If(n== -1 or n==0 or n==1, 1, If(iseven(n) and n<0, oo, if(iseven(n) and n>0, 2^(n/2)*(n/2)!, If(isodd(n) and n<0, (-1)^n/(1-2n)!!, if(isodd(n) and n>0, n!/2^(n/2)/(n/2)! ))))); doublefactorial(n_):= If(n== -1 or n==0 or n==1, 1, If(iseven(n) and n<0, oo, If(iseven(n) and n>0, 2^(n/2)*(n/2)!, if(isodd(n) and n>0, 2^((n-1)/2)*gamma((n-1)/2+1/2)/sqrt(pi) ))));