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 A270929 Numbers k such that (16*10^k - 31)/3 is prime. 496
 1, 2, 3, 4, 15, 20, 24, 32, 38, 40, 63, 93, 104, 194, 208, 514, 535, 600, 928, 1300, 1485, 1780, 2058, 3060, 3356, 3721, 6662, 11552, 15482, 23000, 27375, 34748, 57219, 61251, 85221, 99656, 103214, 103244 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 5 followed by k-2 occurrences of the digit 3 followed by the digits 23 is prime (see Example section). a(39) > 2*10^5. LINKS Makoto Kamada, Search for 53w23. EXAMPLE 3 is in this sequence because (16*10^3 - 31)/3 = 5323 is prime. Initial terms and primes associated: a(1) = 1, 43; a(2) = 2, 523; a(3) = 3, 5323; a(4) = 4, 53323; a(5) = 15, 5333333333333323; a(6) = 20, 533333333333333333323, etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(16*10^# - 31)/3] &] PROG (PARI) isok(n) = ispseudoprime((16*10^n - 31)/3); \\ Michel Marcus, Mar 26 2016 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890. Sequence in context: A173320 A135633 A269725 * A103096 A295757 A005645 Adjacent sequences:  A270926 A270927 A270928 * A270930 A270931 A270932 KEYWORD nonn,base,more AUTHOR Robert Price, Mar 26 2016 EXTENSIONS a(37)-a(38) from Robert Price, Mar 03 2019 STATUS approved

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Last modified January 23 16:48 EST 2020. Contains 331173 sequences. (Running on oeis4.)