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A296443 Numbers k such that 2*10^(2k)-2*10^k+1 is prime. 1
1, 2, 6, 7, 8, 315, 667 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers of this form divide 4*10^(4k)+1.

a(8) > 5000. - Jon E. Schoenfield, Dec 16 2017

LINKS

Table of n, a(n) for n=1..7.

EXAMPLE

181, 19801, 1999998000001, 199999980000001, and 19999999800000001 are prime, while 1998001=277*7213, 199980001=13*41*457*821, and 19999800001=53*5953*63389.

MATHEMATICA

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

PROG

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

CROSSREFS

See A296444 for 2*10^(2k)+2*10^k+1.

Sequence in context: A157671 A196747 A261691 * A102046 A019913 A299421

Adjacent sequences:  A296440 A296441 A296442 * A296444 A296445 A296446

KEYWORD

nonn,more

AUTHOR

Patrick A. Thomas, Dec 13 2017

EXTENSIONS

a(6)-a(7) from Michel Marcus, Dec 13 2017

STATUS

approved

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Last modified July 23 11:44 EDT 2019. Contains 325254 sequences. (Running on oeis4.)