A089017 n for which the number consisting of a string of n 3's and a terminal 1 is not prime.

8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 78, 79, 80, 81, 82

Offset: 1

Author

Lekraj Beedassy, Nov 04 2003

Keywords

• base
• nonn

Comments

Complement of A055520(n)=A055557(n) - 1. The first n for which {10^(n+1) - 7}/3 is composite is thus n=8,corresponding to 333333331=17*19607843.

Crossrefs

Cf. A055520, A089018.

Programs

Mathematica

Select[Range[2,90],!PrimeQ[FromDigits[PadLeft[{1},#,3]]]&]-1 (* Harvey P. Dale, Jun 19 2012 *)

PARI

is(n)=ispseudoprime((10^(n+1)-7)/3) \\ Charles R Greathouse IV, Oct 23 2013