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 A181530 Greatest k <= n such that 3^n + 3^k - 1 is prime, or 0 if no such prime exists. 2
 1, 2, 3, 3, 3, 5, 7, 8, 6, 8, 8, 12, 7, 6, 12, 15, 8, 13, 13, 20, 18, 20, 23, 22, 20, 8, 27, 26, 22, 26, 21, 30, 27, 20, 35, 26, 28, 28, 35, 38, 39, 22, 37, 38, 12, 45, 39, 36, 3, 30, 49, 49, 48, 37, 28, 56, 42, 53, 17, 58, 36, 62, 59, 55, 40, 26, 64, 68, 58, 61 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Pierre CAMI, Table of n, a(n) for n = 1..2000 EXAMPLE For n = 4, 3^4 + 3^4 - 1 = 161 = 7*23, 3^4 + 3^3 - 1 = 107 so a(4) = 3. For n = 154, for k = 0, 1, 2, 3, ..., 154, the numbers 3^154 + 3^k - 1 are respectively divisible by 3, 5531, 73, 5, 7, 19001, 6553, 5, 239, 3541, 7, 5, 33247, 71, 19, 5, 7, 29, 1973, 5, 436467739, 71161, 7, 5, 4283, 37, 73, 5, 7, 11177, 13721, 5, 19, 29207, 7, 5, 64849, 4001, 73, 5, 7, 31, 227009113, 5, 139, 29, 7, 5, 71, 107102231, 19, 5, 7, 10765021647412056860623883, 521, 5, 241, 1448445976112887644909473, 7, 5, 5657, 37, 73, 5, 7, 110661029, 65963, 5, 19, 20411, 7, 5, 331, 29, 73, 5, 7, 7671791, 269, 5, 563, 211, 7, 5, 6553, 113, 19, 5, 7, 425679689, 1301, 5, 334244063, 53, 7, 5, 15254167, 37, 73, 5, 7, 29, 3391, 5, 19, 7727, 7, 5, 4799, 2269, 73, 5, 7, 1822597729, 47, 5, 242496218184092003, 38971, 7, 5, 9994245487379630507640393493999, 9104413, 19, 5, 7, 3581, 1039, 5, 181, 29, 7, 5, 2472681219552827727900539, 37, 73, 5, 7, 47, 99986141, 5, 19, 1237, 7, 5, 55817, 53, 73, 5, 7, 1033, 9187, 5, 199, 71, 7. So a(154) = 0. PROG (PARI) a(n) = my(k=n); while (!isprime(3^n + 3^k - 1), k--; if (k==0, return (0))); k; \\ Michel Marcus, Sep 16 2019 (MAGMA) sol:=[]; for n in [1..70] do k:=n; while not IsPrime(3^n+3^k-1) and k gt 0 do k:=k-1; end while; if k ge 0 then Append(~sol, k); else Append(~sol, 0); end if; end for; sol; // Marius A. Burtea, Sep 16 2019 CROSSREFS Sequence in context: A036020 A036024 A036029 * A035362 A042957 A264870 Adjacent sequences:  A181527 A181528 A181529 * A181531 A181532 A181533 KEYWORD nonn AUTHOR Pierre CAMI, Jan 29 2011 STATUS approved

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Last modified October 14 07:31 EDT 2019. Contains 327995 sequences. (Running on oeis4.)