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A097684
Numbers k such that (10^k - 1)/9 + 6 is prime.
3
1, 2, 4, 5, 8, 23, 29, 40, 131, 136, 215, 611, 767, 2153, 2576, 22973, 42689, 85712, 85864, 112067, 538508, 631715
OFFSET
1,2
COMMENTS
Values indicate primes of the form "(k-1) ones followed by a seven". Related to the base-10 repunit primes.
Some of the larger entries may only correspond to probable primes.
a(20) > 10^5. - Robert Price, Jan 11 2015
Corresponding primes are equal to (10^k + 53)/9. - Robert Price, Jan 11 2015
a(23) > 6.7*10^5, determined by Rytis Slatkevičius. - Jeppe Stig Nielsen, Jan 20 2023
FORMULA
a(n) = A056655(n) + 1 for all n >= 0.
MATHEMATICA
Do[ If[ PrimeQ[(10^n - 1)/9 + 6], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 14 2004 *)
PROG
(PARI) for (i=1, 1000, if(isprime((10^i-1)/9 + 6), print1(i, ", "))) \\ Mohammed Bouayoun (mohammed.bouayoun(AT)sanef.com), Aug 23 2004
CROSSREFS
KEYWORD
nonn
AUTHOR
Carl R. White, Aug 19 2004
EXTENSIONS
a(12)-a(13) from Mohammed Bouayoun (mohammed.bouayoun(AT)sanef.com), Aug 23 2004
a(12)-a(15) from Rick L. Shepherd, Aug 23 2004
a(16)-a(19) from Robert Price, Jan 11 2015
a(20) from Serge Batalov and a(21)-a(22) from Rytis Slatkevičius communicated by Jeppe Stig Nielsen, Jan 20 2023
STATUS
approved