ProjectEuler 383 Divisibility comparison between factori - 拼圖

383. Divisibility comparison between factorials

http://projecteuler.net/problem=383

令 f_5(n) 表示最大的整數 x 使 5^x 能整除 n。

例如 f_5(625000) = 7.

令 T_5(n) 表示滿足 f_5((2*i-1)!) < 2*f_5(i!) 且 1≦i≦n 的 i 的個數。

可以驗證 T_5(10^3) = 68 and T_5(10^9) = 2408210。

求 T_5(10^18)。

--
'Oh, Harry, don't you see?' Hermione breathed. 'If she could have done
one thing to make absolutely sure that every single person in this school
will read your interview, it was banning it!'
---'Harry Potter and the order of the phoenix', P513

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