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 A123608 #9 Jun 02 2025 15:58:57
%S A123608 24,25,27,32,34,38,45,49,55,57,62,64,76,77,80,84,85,87,91,92,93,94,
%T A123608 104,110,115,117,118,121,122,123,124,129,132,133,142,143,144,145,147,
%U A123608 152,154,159,160,161,164,169,170,171,175,177,182,184,185,187,188,195,201
%N A123608 Numbers k such that k, k+1 and 2*k+1 are composite.
%C A123608 Also numbers n such that A098735(n) = 1. Also numbers n such that A098077(n) divides n!. Note that n, n+1 and 2n+1 are all primes only for n = 2.
%H A123608 Amiram Eldar, <a href="/A123608/b123608.txt">Table of n, a(n) for n = 1..10000</a>
%t A123608 Select[Range[500],!PrimeQ[ # ]&&!PrimeQ[ #+1]&&!PrimeQ[2#+1]&]
%t A123608 Select[Range[250],AllTrue[{#,#+1,2#+1},CompositeQ]&] (* _Harvey P. Dale_, Jun 02 2025 *)
%Y A123608 Cf. A098735, A098077.
%K A123608 nonn
%O A123608 1,1
%A A123608 _Alexander Adamchuk_, Nov 15 2006