641

This number is a prime.

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Replacing each digit d of 641 with d copies of the digit d produces another prime, i.e., 641 becomes 66666644441, which is also prime. If we apply the same transform using 66666644441, yet another prime is formed. [Honaker]

Submitted: 1999-09-22 20:12:53;   Last Modified: 2009-03-04 19:01:06.
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