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 A231776 Least positive integer k <= n with (2^k + k) * n - 1 prime, or 0 if such a number k does not exist. 2
 1, 1, 2, 1, 2, 1, 2, 1, 2, 1, 6, 2, 10, 1, 2, 1, 2, 1, 2, 1, 4, 2, 2, 1, 2, 8, 6, 1, 2, 1, 4, 2, 2, 1, 8, 1, 4, 1, 2, 2, 14, 2, 2, 1, 2, 1, 2, 6, 2, 1, 4, 2, 2, 3, 8, 1, 6, 1, 2, 1, 8, 5, 4, 1, 2, 1, 2, 6, 42, 2, 6, 2, 4, 2, 2, 1, 2, 1, 4, 1, 4, 2, 8, 1, 2, 1, 2, 1, 6, 1, 8, 20, 2, 1, 2, 6, 10, 1, 2, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS We find that 75011 is the only value of n <= 10^5 with a(n) = 0. The least positive integer k with (2^k + k)*75011 - 1 prime is 81152. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..10000 EXAMPLE a(3) = 2 since (2^1 + 1) * 3 - 1 = 8 is not prime, but (2^2 + 2) * 3 - 1 = 17 is prime. MATHEMATICA Do[Do[If[PrimeQ[(2^k+k)*n-1], Print[n, " ", k]; Goto[aa]], {k, 1, n}]; Print[n, " ", 0]; Label[aa]; Continue, {n, 1, 100}] lpi[n_]:=Module[{k=1}, While[!PrimeQ[n(2^k+k)-1], k++]; k]; Array[lpi, 100] (* Harvey P. Dale, Aug 10 2019 *) CROSSREFS Cf. A000040, A000079, A231201, A231557, A231561, A231725. Sequence in context: A161299 A161274 A160978 * A055734 A295660 A193169 Adjacent sequences: A231773 A231774 A231775 * A231777 A231778 A231779 KEYWORD nonn AUTHOR Zhi-Wei Sun, Nov 13 2013 STATUS approved

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Last modified August 15 01:22 EDT 2024. Contains 375171 sequences. (Running on oeis4.)