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 A005537 Numbers n such that 4*3^n + 1 is prime. (Formerly M0803) 6
 0, 1, 2, 3, 6, 14, 15, 39, 201, 249, 885, 1005, 1254, 1635, 3306, 3522, 9602, 19785, 72698, 233583, 328689, 537918, 887535, 980925, 1154598, 1499606, 1936890, 2016951, 2143374 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS a(27) > 1.5*10^6. - Matthias Baur, Jan 16 2020 a(20) > 2*10^5. - Robert Price, Nov 23 2013 Primes resulting from a(1)-a(19) are confirmed primes (not probable primes) using BLS (N-1/N+1) test in pfgw. - Robert Price, Nov 23 2013 From Matthias Baur, Jan 16 2020: (Start) Double checked to n=2*10^5, tested further to n=1.5*10^6 using the sieve programs newpgen and srsieve and using Jean PennĂ©'s LLR application (BLS (N-1/N+1) test). a(20) was already known in 2005, but was not listed here until 2018 (see Prime Pages link). (End) REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS C. K. Caldwell, The Prime Pages Douglas E. Iannucci, Deng Moujie and Graeme L. Cohen, On Perfect Totient Numbers, J. Integer Sequences, 6 (2003), #03.4.5. P. Loomis, M. Plytage and J. Polhill, Summing up the Euler 'phi' function, The College Mathematics Journal, vol. 39 (2008), pp. 34-42. H. C. Williams and C. R. Zarnke, Some prime numbers of the forms 2*3^n+1 and 2*3^n-1, Math. Comp., 26 (1972), 995-998. MATHEMATICA a[n_]:=If[PrimeQ[4*3^n + 1 ], n]; DeleteCases[Array[a, 40, 0], Null] (* Stefano Spezia, Nov 12 2018 *) PROG (PARI) a(n) = isprime(4*3^n + 1) \\ Michel Marcus, Jul 12 2013 CROSSREFS Sequence in context: A121556 A123041 A078557 * A306600 A282351 A193093 Adjacent sequences:  A005534 A005535 A005536 * A005538 A005539 A005540 KEYWORD nonn,more AUTHOR EXTENSIONS a(15)-a(17) from Douglas Burke (dburke(AT)nevada.edu) a(18) from Mohammed Bouayoun (Mohammed.Bouayoun(AT)sanef.com), Jan 26 2004 a(19) from Robert Price, Nov 23 2013 a(20)-a(21) from Matthias Baur, Nov 07 2018 a(22) from Matthias Baur, Dec 06 2018 a(23)-a(24) from Matthias Baur, Jul 23 2019 a(25) from Matthias Baur, Dec 07 2019 a(26) from Matthias Baur, Jan 16 2020 a(27)-a(29) from Ryan Propper, May 08 2020 STATUS approved

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Last modified October 20 23:28 EDT 2020. Contains 337910 sequences. (Running on oeis4.)