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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

Table of n, a(n) for n=1..25.

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: A248077 A239547 A060728 * 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 May 21 02:48 EDT 2019. Contains 323434 sequences. (Running on oeis4.)