[R] 階乘

階乘-factorial: 是所有小於或等於該數的正整數的積。
自然數n的階乘寫作n!

(1) factorial(x)
x! = factorial(x) = 1 * 2 * .. * x = Gamma(1 + x)
lfactorial(x)

(2) The binomial distribution with size = n and prob = p has density
p(x) = choose(n,x) p^x (1-p)^(n-x)

(3) combn