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A096508

Numbers k for which 8*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 14, 17, 35, 4175, 4472, 9812, 12260, 12341, 13760, 14576, 53411, 144683, 148328

OFFSET

1,1

COMMENTS

Also numbers k such that (8*10^k + 1)/9 is prime.

a(15) > 2*10^5. - Robert Price, Sep 06 2014

LINKS

FORMULA

a(n) = A056663(n) + 1.

EXAMPLE

35 is a term because 88888888888888888888888888888888889 (34 8's) is a prime number.

MAPLE

select(n -> isprime((8*10^n+1)/9), [$1..10000]); # Robert Israel, Sep 07 2014

MATHEMATICA

Do[ If[ PrimeQ[ 8(10^n - 1)/9 + 1], Print[n]], {n, 0, 30000}] (* Robert G. Wilson v, Oct 15 2004 *)

PROG

(PARI)

for(n=1, 10^4, if(ispseudoprime(8*(10^n-1)/9+1), print1(n, ", "))) \\ Derek Orr, Sep 06 2014

CROSSREFS

KEYWORD

nonn

AUTHOR

Labos Elemer, Jul 12 2004

EXTENSIONS

Four missing terms (9812, 12260, 12341, 13760) added, and a(12)-a(14) added from Kamada data, by Robert Price, Sep 06 2014

STATUS

approved

URL: https://oeis.org/A096508