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 A271585 Numbers k such that (7*10^k + 143)/3 is prime. 0
 1, 2, 3, 6, 7, 10, 11, 25, 26, 32, 122, 123, 126, 161, 292, 320, 743, 1630, 2738, 3178, 4814, 4833, 5030, 7035, 8151, 12554, 13954, 15113, 80490, 96112, 121487, 190683 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For n>1, numbers such that the digit 2 followed by n-2 occurrences of the digit 3 followed by the digits 81 is prime (see Example section). a(33) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 23w81. EXAMPLE 3 is in this sequence because (7*10^3 + 143)/3 = 2381 is prime. Initial terms and primes associated: a(1) = 1, 71; a(2) = 2, 281; a(3) = 3, 2381; a(4) = 6, 2333381; a(5) = 7, 23333381, etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(7*10^# + 143)/3] &] PROG (PARI) is(n)=ispseudoprime((7*10^n+143)/3) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A181498 A030703 A305927 * A285259 A284907 A287241 Adjacent sequences:  A271582 A271583 A271584 * A271586 A271587 A271588 KEYWORD nonn,more AUTHOR Robert Price, Apr 10 2016 EXTENSIONS a(31)-a(32) from Robert Price, Mar 30 2018 STATUS approved

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Last modified December 14 19:27 EST 2019. Contains 329987 sequences. (Running on oeis4.)