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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 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.)