## 2x2 positive integer matrix

### Problem 420

Published on Sunday, 24th March 2013, 01:00 am; Solved by 237; Difficulty rating: 60%A *positive integer matrix* is a matrix whose elements are all positive integers.

Some positive integer matrices can be expressed as a square of a positive integer matrix in two different ways. Here is an example:

We define F(`N`) as the number of the 2x2 positive integer matrices which have a trace less than `N` and which can be expressed as a square of a positive integer matrix in two different ways.

We can verify that F(50) = 7 and F(1000) = 1019.

Find F(10^{7}).