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 A128025 Numbers k such that (8^k - 3^k)/5 is prime. 28
 2, 3, 7, 19, 31, 67, 89, 9227, 43891 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are primes. Verified the first 8 terms in sequence. Also, the next number in the sequence, if one exists is > 43691. - Robert Price, Mar 16 2010 a(10) > 10^5. - Robert Price, Jul 27 2011 LINKS MATHEMATICA k=5; Select[ Prime[ Range[1, 200] ], PrimeQ[ ((k+3)^# - 3^#)/k ]& ] PROG (PARI) is(n)=isprime((8^n-3^n)/5) \\ Charles R Greathouse IV, Feb 17 2017 CROSSREFS Cf. A028491 = numbers n such that (3^n - 1)/2 is prime. Cf. A057468 = numbers n such that 3^n - 2^n is prime. Cf. A059801 = numbers n such that 4^n - 3^n is prime. Cf. A121877 = numbers n such that (5^n - 3^n)/2 is a prime. Cf. A128024, A128026, A128027, A128028, A128029, A128030, A128031, A128032. Sequence in context: A195354 A244638 A113165 * A092064 A152609 A298943 Adjacent sequences:  A128022 A128023 A128024 * A128026 A128027 A128028 KEYWORD hard,more,nonn AUTHOR Alexander Adamchuk, Feb 11 2007 EXTENSIONS 9227 from Farideh Firoozbakht, Apr 08 2007 a(9)=43891 from Robert Price, Jul 27 2011 STATUS approved

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Last modified May 21 01:26 EDT 2019. Contains 323429 sequences. (Running on oeis4.)