ここに一次導関数がある：
f'(k) = d/dk [k * f(k-1) + an] = f(k-1) + k * f'(k-1)
これが2次微分である：
f''(k) = f'(k-1) + k * f''(k-1)
Here is the first derivative:
f'(k) = d/dk [k * f(k-1) + an] = f(k-1) + k * f'(k-1)
Here is the second derivative:
f''(k) = f'(k-1) + k * f''(k-1)