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 A278336 Numbers k such that (94*10^k - 7) / 3 is prime. 0
 0, 1, 4, 5, 9, 16, 20, 41, 43, 63, 127, 159, 413, 1591, 1812, 2031, 2315, 2437, 4177, 4860, 5771, 7060, 7389, 9925, 34103, 115879 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS For k > 0, numbers such that the digits 31 followed by k-1 occurrences of the digit 3 followed by the digit 1 is prime (see Example section). a(27) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 313w1. EXAMPLE 4 is in this sequence because (94*10^4 - 7) / 3 = 313331 is prime. Initial terms and primes associated: a(1) = 0, 29; a(2) = 1, 311; a(3) = 4, 313331; a(4) = 5, 3133331; a(5) = 9, 31333333331; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(94*10^# - 7) / 3] &] PROG (PARI) is(n)=ispseudoprime((94*10^n - 7)/3) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A020682 A243702 A155565 * A051216 A255978 A069089 Adjacent sequences:  A278333 A278334 A278335 * A278337 A278338 A278339 KEYWORD nonn,more,hard AUTHOR Robert Price, Nov 18 2016 EXTENSIONS a(26) from Robert Price, Mar 10 2020 STATUS approved

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Last modified September 22 12:59 EDT 2021. Contains 347607 sequences. (Running on oeis4.)