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 A295988 Numbers n such that (10^n)/2 - 1 is prime. 0
 3, 4, 5, 7, 15, 55, 211, 391, 595, 3461, 5029, 5220, 5333, 8073, 15797, 16132, 21457, 29283, 78791, 85143, 179973, 211030, 445774, 464844, 511057 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS FORMULA a(n) = 1 + A056712(n). - Omar E. Pol, Dec 02 2017 EXAMPLE 499, 4999, 49999, 4999999 are prime, while 4, 49, 499999, 49999999 are composite. MATHEMATICA Select[Range[10^3], PrimeQ[10^#/2 - 1] &] (* Michael De Vlieger, Dec 02 2017 *) PROG (PARI) isok(n) = isprime(10^n/2 - 1); \\ Michel Marcus, Dec 02 2017 CROSSREFS Cf. A056712, A093945. Sequence in context: A239547 A060728 A330712 * A216433 A101761 A035359 Adjacent sequences:  A295985 A295986 A295987 * A295989 A295990 A295991 KEYWORD nonn,more AUTHOR Patrick A. Thomas, Dec 02 2017 EXTENSIONS a(7)-a(25) from Michel Marcus and Omar E. Pol, Dec 02 2017 STATUS approved

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Last modified June 17 04:55 EDT 2021. Contains 345080 sequences. (Running on oeis4.)