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 A100317 Numbers k such that exactly one of k - 1 and k + 1 is prime. 7
 1, 2, 3, 8, 10, 14, 16, 20, 22, 24, 28, 32, 36, 38, 40, 44, 46, 48, 52, 54, 58, 62, 66, 68, 70, 74, 78, 80, 82, 84, 88, 90, 96, 98, 100, 104, 106, 110, 112, 114, 126, 128, 130, 132, 136, 140, 148, 152, 156, 158, 162, 164, 166, 168, 172, 174, 178, 182, 190, 194, 196, 200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Beginning with a(2) = 3, n such that exactly one of n - 1 and n + 1 is composite. LINKS G. C. Greubel, Table of n, a(n) for n = 1..10000 EXAMPLE 3 is in the sequence because 2 is prime but 4 is composite. 4 is not in the sequence because both 3 and 5 are prime. 5 is not in the sequence either because both 4 and 6 are composite. MATHEMATICA Select[Range[250], Xor[PrimeQ[# - 1], PrimeQ[# + 1]] &] (* G. C. Greubel, Apr 25 2019 *) Module[{nn=Table[If[PrimeQ[n], 1, 0], {n, 0, 220}], t1, t2}, t1=Mean/@ SequencePosition[ nn, {1, _, 0}]; t2=Mean/@SequencePosition[nn, {0, _, 1}]; Flatten[ Join[t1, t2]]//Sort]-1 (* Harvey P. Dale, Jul 13 2019 *) PROG (PARI) for(n=1, 250, if(isprime(n-1)+isprime(n+1)==1, print1(n, ", "))) (Magma) [n: n in [1..250] | IsPrime(n-1) xor IsPrime(n+1) ]; // G. C. Greubel, Apr 25 2019 (Sage) [n for n in (1..250) if (is_prime(n-1) + is_prime(n+1) == 1)] # G. C. Greubel, Apr 25 2019 CROSSREFS Cf. A100318 (at least one of n - 1 and n + 1 is composite). Cf. A001477, A169546, A171689, A099049, A014574 (no intersection with this sequence). Sequence in context: A132327 A281929 A286092 * A295030 A317655 A060697 Adjacent sequences: A100314 A100315 A100316 * A100318 A100319 A100320 KEYWORD nonn AUTHOR Rick L. Shepherd, Nov 13 2004 STATUS approved

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Last modified June 13 09:03 EDT 2024. Contains 373383 sequences. (Running on oeis4.)