形式幂级数
https://math.stackexchange.com/questions/557230/ordinary-generating-function-for-binom3nn
https://math.stackexchange.com/questions/774434/generating-function-of-binom3nn?noredirect=1
https://math.stackexchange.com/questions/557230/ordinary-generating-function-for-binom3nn
https://math.stackexchange.com/questions/774434/generating-function-of-binom3nn?noredirect=1