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 A281646 Numbers k such that (25*10^k - 241)/9 is prime. 0
 2, 4, 5, 8, 20, 35, 80, 131, 169, 262, 364, 484, 580, 692, 1687, 1834, 3476, 3950, 5963, 6329, 13339, 18617, 72703, 103360 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 7 followed by the digits 51 is prime (see Example section). a(25) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 27w51. EXAMPLE 4 is in this sequence because (25*10^4 - 241)/9 = 27751 is prime. Initial terms and primes associated: a(1) = 2, 251; a(2) = 4, 27751; a(3) = 5, 277751; a(4) = 8, 277777751; a(5) = 20, 277777777777777777751; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[(25*10^# - 241)/9] &] PROG (PARI) isok(k) = ispseudoprime((25*10^k-241)/9); \\ Altug Alkan, May 07 2018 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A253426 A092061 A256760 * A240460 A239405 A097684 Adjacent sequences:  A281643 A281644 A281645 * A281647 A281648 A281649 KEYWORD nonn,more,hard AUTHOR Robert Price, Jan 25 2017 EXTENSIONS a(24) from Robert Price, May 07 2018 STATUS approved

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Last modified March 30 22:55 EDT 2020. Contains 333132 sequences. (Running on oeis4.)