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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 (list; graph; refs; listen; history; text; internal format)
 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 LINKS Makoto Kamada, Prime numbers of the form 11...117. 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 Cf. A004023, A056655, A097683, A097685. Sequence in context: A281646 A240460 A239405 * A277289 A218347 A171411 Adjacent sequences:  A097681 A097682 A097683 * A097685 A097686 A097687 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 STATUS approved

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Last modified April 8 01:54 EDT 2020. Contains 333312 sequences. (Running on oeis4.)