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 A128024 Numbers n such that (7^n - 3^n)/4 is prime. 29
 3, 7, 19, 109, 131, 607, 863, 2917, 5923, 12421 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are primes. No other terms < 100,000. LINKS MATHEMATICA k=4; Select[ Prime[ Range[1, 200] ], PrimeQ[ ((k+3)^# - 3^#)/k ]& ] PROG (PARI) forprime(p=3, 1e5, if(ispseudoprime((7^p-3^p)/4), print1(p", "))) \\ Charles R Greathouse IV, Jun 01 2011 (Python) from sympy import isprime def aupto(lim): return [k for k in range(lim+1) if isprime((7**k-3**k)//4)] print(aupto(900)) # Michael S. Branicky, Mar 07 2021 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. A128025, A128026, A128027, A128028, A128029, A128030, A128031, A128032. Sequence in context: A215487 A146037 A172514 * A245723 A118128 A113875 Adjacent sequences:  A128021 A128022 A128023 * A128025 A128026 A128027 KEYWORD hard,more,nonn AUTHOR Alexander Adamchuk, Feb 11 2007 EXTENSIONS a(8)-a(9) from Farideh Firoozbakht, Apr 08 2007 a(10) from Robert Price, Jun 01 2011 STATUS approved

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Last modified January 21 00:46 EST 2022. Contains 350473 sequences. (Running on oeis4.)