A179005 Indices of nonprime repunits: nonnegative numbers n such that 11...111 = (10^n - 1)/9 is composite.
0, 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69
Offset: 1
Keywords
Examples
3 appears because the 3-digit repunit 111 is composite (37*3). 4 appears because the 4-digit repunit 1111 is composite (101*11).
Programs
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PARI
isok(n) = !isprime((10^n - 1)/9); \\ Michel Marcus, Sep 05 2013
Comments