题目216：考查形如2n^2-1的数字的素数性质

欧拉计划

Investigating the primality of numbers of the form 2n²-1

Consider numbers t(n) of the form t(n) = 2n²-1 with n > 1.
The first such numbers are 7, 17, 31, 49, 71, 97, 127 and 161.
It turns out that only 49 = 7*7 and 161 = 7*23 are not prime.
For n ≤ 10000 there are 2202 numbers t(n) that are prime.

How many numbers t(n) are prime for n ≤ 50,000,000 ?

题目：

对于形如 t(n) = 2n²-1， n > 1 的数字 t(n) 来说，前几个分别是 7, 17, 31, 49, 71, 97, 127 和 161，可知，只有 49 = 7*7 以及 161 = 7*23 不是素数。在 n ≤ 10000 的范围内，一共有 2202 个 t(n) 是素数。

请问，在 n ≤ 50,000,000 范围内，有多少个 t(n) 是素数？