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.

(There is one curio for this number that has not yet been approved by an editor.)

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