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 A280449 Numbers n such that (2*10^n - 71)/3 is prime. 0
 2, 3, 5, 6, 8, 11, 15, 25, 38, 54, 101, 105, 158, 295, 303, 389, 482, 558, 693, 1965, 3503, 9818, 13255, 30351, 51285, 54767, 57384, 88808, 109645, 174301, 187062 (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 43 is prime (see Example section). a(32) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 6w43. EXAMPLE 5 is in this sequence because (2*10^5 - 71) / 3 = 66643 is prime. Initial terms and primes associated: a(1) = 2, 43; a(2) = 3, 643; a(3) = 5, 66643; a(4) = 6, 666643; a(5) = 8, 66666643; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[(2*10^# - 71) / 3] &] PROG (PARI) is(n)=isprime((2*10^n - 71)/3) \\ Charles R Greathouse IV, Jan 03 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A117517 A339732 A098491 * A107947 A120768 A271109 Adjacent sequences: A280446 A280447 A280448 * A280450 A280451 A280452 KEYWORD nonn,more,hard AUTHOR Robert Price, Jan 03 2017 EXTENSIONS a(29) - a(31) from Robert Price, Dec 31 2017 STATUS approved

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Last modified January 26 16:14 EST 2023. Contains 359833 sequences. (Running on oeis4.)