25169

This number is a prime.

The smallest prime formed by concatenating the squares (9, 16, 25) of a primitive Pythagorean triple in the reverse order. Note the prime sum of digits and the fact that this Pythagorean triple (X, Y, Z) is the only one that forms with its reversal a palindromic prime XYZYX (34543) with also a prime sum of digits. [Beedassy]