6569

This number is a prime.

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The largest of only three primes formed from the concatenation in order of two double-digit composite numbers differing by 4. The other are 3539 and 4549. Shall we call the pairs of composite numbers differing by 4, i.e., (35, 39), (45, 49), (65, 69), pairs of “cousin” composite numbers? [Loungrides]