二項係数の和とフィボナッチ
https://gyazo.com/b2df8c9ce8cf3c362ec03a2b2839fd8b
$ \sum_i \binom{N-i}{i} = F_N
where $ F_0 = F_1 = 1, F_n = F_{n-2} + F_{n-1}
https://gyazo.com/68fc51e0aad6ed0f251979427ce9fbfe
$ F_N = \sum_{i\ge 0} \binom{N-i}{i} = 1 + \sum_{i\ge 1} \binom{N-i}{i}
$ = 1 + \sum_{i \ge 1} \left(\binom{N-i - 1}{i} + \binom{N-i - 1}{i-1}\right)
$ = 1 + \sum_{i \ge 1} \binom{N-i - 1}{i} + \sum_{i \ge 1} \binom{N-i - 1}{i-1}
$ = 1 + \sum_{i \ge 1} \binom{N-i - 1}{i} + \sum_{j \ge 0} \binom{N - j - 2}{j}
$ = \sum_{i \ge 0} \binom{N-i - 1}{i} + \sum_{j \ge 0} \binom{N - j - 2}{j}
$ = \sum_{i \ge 0} \binom{(N-1) - i}{i} + \sum_{i \ge 0} \binom{(N-2) - i}{i}
$ = F_{N-1} + F_{N-2}