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 A261577 Numbers m such that (4^m + 11) / 3 is prime. 1
 1, 4, 10, 34, 40, 106, 418, 682, 12702, 30484, 182026, 217720, 241306 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS From Bruno Berselli, Aug 26 2015: (Start) After 1, m is always even (for m odd 4^m+11 is divisible by 5). Let m = 2*h. For h = 3*k+1, 9*k+3, 11*k+2, 11*k+8, 13*k+8, 19*k+6, 23*k+10, 23*k+14 and 29*k+28, 4^m+11 is divisible by 9, 37, 89, 23, 53, 229, 47, 1013 and 59, respectively. (End) All terms appear to be of the form 3*k+1. - Dhilan Lahoti, Aug 31 2015 12702 is the first counterexample to Dhilan Lahoti's conjecture: 12702 = 3*4234. - Bruno Berselli, Feb 02 2017 a(14) > 300,000. - Robert Price, Mar 18 2017 LINKS Table of n, a(n) for n=1..13. EXAMPLE 4 is in the sequence because (4^4+11)/3 = 89 is prime. 10 is in the sequence because (4^10+11)/3 = 349529 is prime. MATHEMATICA Select[Range[0, 5000], PrimeQ[(4^# + 11)/3] &] PROG (Magma) [n: n in [0..1500] | IsPrime((4^n+11) div 3)]; (PARI) is(n)=isprime((4^n + 11) / 3) \\ Anders Hellström, Aug 31 2015 CROSSREFS Cf. A163868. Cf. similar sequences listed in A261539. Sequence in context: A149172 A338296 A105680 * A224217 A066454 A301595 Adjacent sequences: A261574 A261575 A261576 * A261578 A261579 A261580 KEYWORD nonn,more AUTHOR Vincenzo Librandi, Aug 26 2015 EXTENSIONS a(9)-a(12) from Robert Price, Feb 01 2017 a(13) from Robert Price, Mar 18 2017 STATUS approved

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Last modified August 10 12:00 EDT 2024. Contains 375056 sequences. (Running on oeis4.)