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A296446 Numbers k such that 10^(2k)/2 + 10^k + 1 are prime. 1
1, 2, 3, 4, 7, 10, 56, 154, 170 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

a(10) > 5000. - Jon E. Schoenfield, Dec 16 2017

LINKS

Table of n, a(n) for n=1..9.

EXAMPLE

61, 5101, 501001, 50010001, and 50000000010000000001 are prime, while 5000100001=13*1597*240841, 500001000001=2357*212134493, 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 A296445 for 10^(2k)/2 - 10^k + 1.

Sequence in context: A094862 A104722 A270613 * A226387 A102282 A232543

Adjacent sequences:  A296443 A296444 A296445 * A296447 A296448 A296449

KEYWORD

nonn,more

AUTHOR

Patrick A. Thomas, Dec 13 2017

EXTENSIONS

a(8)-a(9) from Michel Marcus, Dec 13 2017

STATUS

approved

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Last modified June 18 18:52 EDT 2019. Contains 324215 sequences. (Running on oeis4.)