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 A068780 Composite numbers n such that n+1 is also composite. 13
 8, 9, 14, 15, 20, 21, 24, 25, 26, 27, 32, 33, 34, 35, 38, 39, 44, 45, 48, 49, 50, 51, 54, 55, 56, 57, 62, 63, 64, 65, 68, 69, 74, 75, 76, 77, 80, 81, 84, 85, 86, 87, 90, 91, 92, 93, 94, 95, 98, 99, 104, 105, 110, 111, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For all primes p, neither p nor p-1 is in the sequence. - Jon Perry, Oct 12 2014 LINKS J. Stauduhar, Table of n, a(n) for n = 1..10000 FORMULA There are x - 2x/log x + O(x/log^2 x) members up to x. The coefficient of the next asymptotic term depends on the quantitative version of the twin prime conjecture (though it can be bounded between -0.6796763684 and 2.4885722184, with the former conjectured to be the case). - Charles R Greathouse IV, Dec 19 2018 MAPLE q:= n-> andmap(not isprime, [n, n+1]): select(q, [\$1..150])[]; # Alois P. Heinz, Jun 24 2021 MATHEMATICA Select[ Range[2, 200], !PrimeQ[ # ] && !PrimeQ[ # + 1] &] PROG (Magma) [n: n in [1..200] | not IsPrime(n) and not IsPrime(n+1)]; // Vincenzo Librandi, Oct 17 2014 (PARI) is(n)=!isprime(n) && !isprime(n+1) \\ Charles R Greathouse IV, Dec 19 2018 CROSSREFS Cf. A001359. Equals A005381(n) - 1. Sequence in context: A308893 A083134 A299143 * A174041 A096784 A175859 Adjacent sequences: A068777 A068778 A068779 * A068781 A068782 A068783 KEYWORD nonn,easy AUTHOR Robert G. Wilson v, Mar 04 2002 EXTENSIONS Definition reworded by N. J. A. Sloane, Aug 24 2012 STATUS approved

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Last modified December 8 13:46 EST 2023. Contains 367679 sequences. (Running on oeis4.)