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 A276846 Numbers k such that (4*10^k + 143) / 3 is prime. 0
 1, 2, 3, 4, 7, 9, 15, 21, 22, 44, 49, 53, 63, 127, 145, 393, 856, 1006, 1883, 2263, 5684, 13324, 14291, 27435, 38897, 114076 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For n>1, numbers such that the digit 1 followed by n-2 occurrences of the digit 3 followed by the digits 81 is prime (see Example section). a(27) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 13w81. EXAMPLE 2 is in this sequence because (4*10^2 + 143) / 3 = 1381 is prime. Initial terms and primes associated: a(1) = 1, 61; a(2) = 2, 181; a(3) = 3, 1381; a(4) = 4, 13381; a(5) = 7, 13333381, etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(4*10^# + 143) / 3] &] PROG (PARI) is(n) = ispseudoprime((4*10^n + 143) / 3); \\ Altug Alkan, Sep 20 2016 (MAGMA) [n: n in [0..500] | IsPrime((4*10^n+143) div 3)]; // Vincenzo Librandi, Sep 22 2016 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A317885 A321535 A108809 * A303665 A027947 A324736 Adjacent sequences:  A276843 A276844 A276845 * A276847 A276848 A276849 KEYWORD nonn,more AUTHOR Robert Price, Sep 20 2016 EXTENSIONS a(26) from Robert Price, Mar 05 2018 STATUS approved

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Last modified June 19 06:47 EDT 2019. Contains 324218 sequences. (Running on oeis4.)