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 A280431 Numbers k such that (2*10^k - 41)/3 is prime. 0
 2, 3, 4, 5, 8, 9, 27, 32, 39, 44, 101, 140, 268, 386, 695, 741, 1155, 1564, 1611, 4076, 6083, 11672, 26685, 50603, 134720 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For n>1, numbers such that n-2 occurrences of the digit 6 followed by the digits 53 is prime (see Example section). a(26) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 6w53. EXAMPLE 4 is in this sequence because (2*10^4 - 41) / 3 = 6653 is prime. Initial terms and primes associated: a(1) = 2, 53; a(2) = 3, 653; a(3) = 4, 6653; a(4) = 5, 66653; a(5) = 8, 66666653; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[(2*10^# - 41) / 3] &] PROG (PARI) is(n)=ispseudoprime((2*10^n-41)/3) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A152606 A057911 A085266 * A056804 A305399 A101547 Adjacent sequences:  A280428 A280429 A280430 * A280432 A280433 A280434 KEYWORD nonn,more,hard AUTHOR Robert Price, Jan 02 2017 EXTENSIONS a(25) from Robert Price, Feb 14 2018 STATUS approved

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Last modified August 4 11:13 EDT 2021. Contains 346447 sequences. (Running on oeis4.)