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 A297422 Numbers k such that 4*10^(2k) + 2*(10^k) + 1 is prime. 2
 0, 1, 3, 5, 10, 196, 251, 310, 415, 1107, 2422 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Numbers of this form divide 8*(10^3k)-1. a(12) > 50000. - Robert Price, Apr 11 2018 LINKS EXAMPLE 7, 421, 4002001, and 40000200001 are prime, while 40201=7*5743, 400020001=31*12903871, etc. MAPLE select(k->isprime(4*10^(2*k)+2*(10^k)+1), [\$0..1000]); # Muniru A Asiru, Apr 11 2018 MATHEMATICA Select[Range[1500], PrimeQ[4 (10^(2 #)) + 2 (10^#) + 1] &] (* Michael De Vlieger, Dec 30 2017 *) PROG (PARI) isok(k) = isprime(4*10^(2*k)+2*(10^k)+1); \\ Michel Marcus, Dec 30 2017 CROSSREFS Sequence in context: A317338 A305510 A024329 * A064523 A268034 A286567 Adjacent sequences:  A297419 A297420 A297421 * A297423 A297424 A297425 KEYWORD nonn,more AUTHOR Patrick A. Thomas, Dec 30 2017 EXTENSIONS a(6)-a(10) from Michael De Vlieger, Dec 30 2017 a(11) from Jon E. Schoenfield, Jan 01 2018 STATUS approved

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Last modified October 17 19:44 EDT 2019. Contains 328128 sequences. (Running on oeis4.)