19191...19191 (25-digits)

This number is a prime.

                                                 19191 9191917161 0101619191

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Change every second digit (which are all 1s) to either 3, 7, or 9, and the result remains prime. Remove all of the 1s and you are left with the largest 12-digit prime with this property. [Vrba]

Submitted: 2006-12-02 09:28:36;   Last Modified: 2008-01-30 11:28:00.
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