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A089017 n for which the number consisting of a string of n 3's and a terminal 1 is not prime. 3
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

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.

LINKS

Table of n, a(n) for n=1..70.

MATHEMATICA

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

PROG

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

CROSSREFS

Cf. A055520, A089018.

Sequence in context: A168098 A020722 A046037 * A182361 A023391 A115419

Adjacent sequences:  A089014 A089015 A089016 * A089018 A089019 A089020

KEYWORD

base,nonn

AUTHOR

Lekraj Beedassy, Nov 04 2003

STATUS

approved

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Last modified July 18 19:00 EDT 2019. Contains 325144 sequences. (Running on oeis4.)