6521

This number is a prime.

π(6521) = 6! + 5! + 2! + 1!. There is only one other prime number with this property. [Honaker]

The largest prime from a cyclic permutation on a side of the prime cube, i.e., the unique arrangement (ignoring rotations or reflections) of eight consecutive digits on a cube's vertices such that any two adjacent digits of an edge sum to a prime. [Beedassy]

"Project 6521" is a special task force within the Communist Party system established to increase security around sensitive anniversaries. [Yongkang]

6521 and its prime index (843) are the largest such pair whose sum, difference, product, and concatenation are all distinct-digit numbers. [Gaydos]