A105427 Numbers n such that the near-repdigit number consisting of a 1 followed by n 3's (i.e., of form 1333...33) is composite.

2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 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, 70, 71, 72, 73, 74, 75

Offset: 1

Author

Lekraj Beedassy, Apr 08 2005

Keywords

• nonn

Comments

Complement of A056698.

Links

• FactorDB, Factorizations of (10^n-1)/3+10^n
• Makoto Kamada, Factorizations of 133...33.

Crossrefs

Cf. A051200, A089017, A093671.

Programs

Mathematica

Select[Range[100],CompositeQ[FromDigits[PadRight[{1},#,3]]]&]-1 (* Harvey P. Dale, Jul 23 2014 *)

PARI

isok(n) = ! isprime(10^n+(10^n-1)/3) \\ Michel Marcus, Jul 28 2013