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 A141274 #19 Jul 19 2025 14:52:07 %S A141274 12,28,36,60,100,130,138,150,172,180,198,222,270,292,348,396,420,432, %T A141274 456,520,540,568,612,618,640,682,732,768,786,820,838,880,928,996,1032, %U A141274 1060,1068,1090,1096,1212,1222,1276,1300,1306,1320,1372,1428,1486,1548 %N A141274 Composite numbers k such that k+1 is prime and the index in the composites is also prime. %C A141274 Suggested by Carlos Rivera's Puzzle 449. %H A141274 Michael S. Branicky, <a href="/A141274/b141274.txt">Table of n, a(n) for n = 1..10000</a> %H A141274 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_449.htm">Puzzle 449. Count N composites after p</a>, The Prime Puzzles & Problems Connection. %F A141274 Compute the composite numbers beginning with 4. At 4 the composite index is zero, at 6 it is 1, at 8 it is 2, ... and at 12 it is 5, etc. %e A141274 a(1) = 12 because 12+1 = 13, prime and the index of 12 is 5, so both the composite 12+1 and the index are prime. %t A141274 Select[Range[4,1548],!PrimeQ[#]&&PrimeQ[#+1]&&PrimeQ[#-PrimePi[#]-2]&] (* _James C. McMahon_, Jul 19 2025 *) %o A141274 (Python) %o A141274 from sympy import compositepi, isprime %o A141274 def ok(n): %o A141274 return not isprime(n) and isprime(n+1) and isprime(compositepi(n)-1) %o A141274 print([k for k in range(1549) if ok(k)]) # _Michael S. Branicky_, Dec 28 2021 %Y A141274 Cf. A141275. %K A141274 easy,nonn %O A141274 1,1 %A A141274 _Enoch Haga_, Jun 21 2008 %E A141274 Edited by _N. J. A. Sloane_, Jun 21 2008