$H^n_k = \binom{n + k - 1}{k} = \frac{(n + k - 1)!}{k!(n - 1)!}$
重複組合せの上下反転 $H^n_k = \binom{n + k - 1}{k} = \binom{n + k - 1}{n - 1} = H^k_{n-1}$
combination with repetition combination with replacement comb_rep multi-choose