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 A282194 a(n) = smallest positive k such that 2*n + 2^k + 1 is composite. 1
 3, 5, 2, 1, 4, 2, 1, 7, 2, 1, 2, 1, 1, 3, 2, 1, 1, 2, 1, 4, 2, 1, 2, 1, 1, 2, 1, 1, 3, 2, 1, 1, 2, 1, 3, 2, 1, 1, 2, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 4, 2, 1, 4, 2, 1, 2, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 3, 2, 1, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 2, 1, 2, 1, 1, 2, 1, 1, 3, 2, 1, 1, 1, 1, 4, 2, 1, 3, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Least k such that a(k) = n are 3, 2, 0, 4, 1, 112, 7, 32917, 802, 9712, 1198673602 for the initial terms. LINKS Altug Alkan, Table of n, a(n) for n = 0..10000 EXAMPLE a(1) = 5 because 3 + 2^k is prime for 0 < k < 5 and 3 + 2^5 = 35 is composite. MATHEMATICA spk[n_]:=Module[{k=1}, While[!CompositeQ[2n+2^k+1], k++]; k]; Array[spk, 110, 0] (* Harvey P. Dale, Apr 26 2017 *) PROG (PARI) a(n) = my(k=1); while(isprime(2*n+2^k+1), k++); k; CROSSREFS Cf. A067076, A067760, A153238, A282026. Sequence in context: A197521 A161865 A145325 * A126353 A094791 A243524 Adjacent sequences:  A282191 A282192 A282193 * A282195 A282196 A282197 KEYWORD nonn AUTHOR Altug Alkan, Feb 15 2017 STATUS approved

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Last modified January 19 21:52 EST 2019. Contains 319310 sequences. (Running on oeis4.)