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 A173978 #24 Sep 12 2019 10:47:28 %S A173978 2,6,9,12,15,18,19,21,24,27,30,33,34,36,39,40,42,45,48,49,51,54,57,60, %T A173978 61,63,64,66,69,72,75,78,79,81,82,84,87,90,93,94,96,99,102,105,106, %U A173978 108,109,111,112,114,117,120,123,124 %N A173978 Numbers n such that the least prime factor of 2n - 3 is less than that of 2n - 1, unless 2n - 3 and 2n - 1 are (twin) primes. %C A173978 Integers > 1 for which 2n - 3 is not in A001359 and A020639(2n-3) < A020639(2n-1). %C A173978 Every multiple of 3 greater than 3 is in the sequence. %H A173978 Amiram Eldar, <a href="/A173978/b173978.txt">Table of n, a(n) for n = 1..1000</a> %e A173978 a(3) = 9 because 2*9 - 3 = 15, the least prime factor of which is 3 and that is smaller than the least prime factor of 2*9 - 1 = 17. %t A173978 Select[Range[200], Not[PrimeQ[2# - 3] && PrimeQ[2# - 1]] && TrueQ[FactorInteger[2# - 3][[1, 1]] < FactorInteger[2# - 1][[1, 1]]] &] (* _Alonso del Arte_, Jun 05 2011 *) %Y A173978 Cf. A020639, A001359, A173977. %K A173978 nonn %O A173978 1,1 %A A173978 _Vladimir Shevelev_, Mar 04 2010 %E A173978 More terms from _Alonso del Arte_, Jun 05 2011