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 A294488 Numbers k such that (73*10^k - 91)/9 is prime. 0
 1, 3, 4, 6, 7, 10, 63, 84, 181, 186, 205, 235, 426, 612, 3379, 4203, 4410, 9091, 12640, 26386, 39069, 45451 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 1 followed by the digits 01 is prime (see Example section). a(23) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 81w01. EXAMPLE 3 is in this sequence because (73*10^3 - 91)/9 = 8101 is prime. Initial terms and primes associated: a(1) = 1, 71; a(2) = 3, 8101; a(3) = 4, 81101; a(4) = 6, 8111101; a(5) = 7, 81111101; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[(73*10^# - 91)/9] &] ParallelMap[If[PrimeQ[(73*10^# - 91)/9], #, Nothing] &, Range@100000] (* Robert G. Wilson v, Oct 31 2017 *) PROG (PARI) isok(n) = isprime((73*10^n - 91)/9); \\ Michel Marcus, Nov 01 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A108588 A194095 A064404 * A335059 A047514 A011975 Adjacent sequences:  A294485 A294486 A294487 * A294489 A294490 A294491 KEYWORD nonn,more,hard AUTHOR Robert Price, Oct 31 2017 STATUS approved

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Last modified June 19 05:18 EDT 2021. Contains 345125 sequences. (Running on oeis4.)