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 A266421 Numbers n such that (2^(n+7)*5^(n+5) - 204979)/9 is prime (n > 0). 3
 5, 11, 127, 149, 199, 239, 503, 1069, 4073 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that '21669' appended to n times the digit 4 is prime. Up to a(9) the terms themselves are primes. a(1), a(2), a(6), a(9), and (2^(a(1)+7) * 5^(a(1)+5) - 204979)/9 = 4444421669 are also Sophie Germain primes. a(10) > 50000 (if it exists). LINKS EXAMPLE 5 appears because 4444421669 ('4' concatenated 5 times and prepended to '21669') is prime. MAPLE A266421:=n->`if`(isprime((2^(n+7) * 5^(n+5) - 204979)/9), n, NULL): seq(A266421(n), n=1..5000); MATHEMATICA Select[ Range[5000], PrimeQ[(2^(# + 7) * 5^(# + 5) - 204979)/9] &] PROG (MAGMA)[n: n in[1 .. 1000] | IsPrime((2^(n+7) * 5^(n+5) - 204979) div 9)]; (PARI) is(n)=isprime((2^(n+7) * 5^(n+5) - 204979)/9) CROSSREFS Cf. A260903, A265629. Sequence in context: A222568 A248253 A262309 * A094108 A222674 A197538 Adjacent sequences:  A266418 A266419 A266420 * A266422 A266423 A266424 KEYWORD nonn,base,hard,more AUTHOR Mikk Heidemaa, Dec 29 2015 STATUS approved

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Last modified October 20 12:47 EDT 2019. Contains 328257 sequences. (Running on oeis4.)