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A296445 Numbers k such that 10^(2k)/2 - 10^k + 1 are prime. 1
1, 5, 10, 12, 24, 48, 960, 2217 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers of this form divide 10^(4k)+4.

LINKS

Table of n, a(n) for n=1..8.

EXAMPLE

41, 4999900001, 49999999990000000001, and 499999999999000000000001 are prime, while 4901=13^2*29, 499001=17*149*197, etc.

MATHEMATICA

ParallelMap[If[PrimeQ[10^(2 #)/2 - 10^# + 1], #, Nothing] &, Range@ 4000] (* Robert G. Wilson v, Dec 13 2017 *)

PROG

(PARI) isok(k) = isprime(10^(2*k)/2-10^k+1); \\ Michel Marcus, Dec 13 2017

CROSSREFS

See A296446 for 10^(2k)/2 + 10^k + 1.

Sequence in context: A103676 A313375 A276606 * A313376 A129846 A194464

Adjacent sequences:  A296442 A296443 A296444 * A296446 A296447 A296448

KEYWORD

nonn,more

AUTHOR

Patrick A. Thomas, Dec 13 2017

EXTENSIONS

a(7) from Michel Marcus, Dec 13 2017

a(8) from Robert G. Wilson v, Dec 13 2017

STATUS

approved

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Last modified June 19 00:55 EDT 2019. Contains 324217 sequences. (Running on oeis4.)