This number is a prime.
The largest prime formed from the difference m m's minus n n's, where m, n are successive digits, i.e., 666666-55555. Note that there are other two such prime differences, i.e., 4444-333=4111 and 333-22=311 (emirp). [Loungrides]
The smallest prime number that does not produce a distinct
digit number when multiplied by any positive integer. That
is, it and all of its multiples each has at least one
duplicated digit. [Gaydos]
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