This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A068780 #28 Feb 01 2025 18:08:13
%S A068780 8,9,14,15,20,21,24,25,26,27,32,33,34,35,38,39,44,45,48,49,50,51,54,
%T A068780 55,56,57,62,63,64,65,68,69,74,75,76,77,80,81,84,85,86,87,90,91,92,93,
%U A068780 94,95,98,99,104,105,110,111,114,115,116,117,118,119,120,121,122,123,124
%N A068780 Composite numbers n such that n+1 is also composite.
%C A068780 For all primes p, neither p nor p-1 is in the sequence. - _Jon Perry_, Oct 12 2014
%H A068780 J. Stauduhar, <a href="/A068780/b068780.txt">Table of n, a(n) for n = 1..10000</a>
%F A068780 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
%p A068780 q:= n-> andmap(not isprime, [n, n+1]):
%p A068780 select(q, [$1..150])[]; # _Alois P. Heinz_, Jun 24 2021
%t A068780 Select[ Range[2, 200], !PrimeQ[ # ] && !PrimeQ[ # + 1] &]
%t A068780 SequencePosition[Table[If[CompositeQ[n],1,0],{n,150}],{1,1}][[;;,1]] (* _Harvey P. Dale_, Feb 01 2025 *)
%o A068780 (Magma) [n: n in [1..200] | not IsPrime(n) and not IsPrime(n+1)]; // _Vincenzo Librandi_, Oct 17 2014
%o A068780 (PARI) is(n)=!isprime(n) && !isprime(n+1) \\ _Charles R Greathouse IV_, Dec 19 2018
%Y A068780 Cf. A001359.
%Y A068780 Equals A005381(n) - 1.
%K A068780 nonn,easy
%O A068780 1,1
%A A068780 _Robert G. Wilson v_, Mar 04 2002
%E A068780 Definition reworded by _N. J. A. Sloane_, Aug 24 2012