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 A269721 #12 Mar 07 2016 12:51:58 %S A269721 1,3,5,7,23,25 %N A269721 Integers k such that k, k+2, k+4 and k+6 are prime powers (A000961). %C A269721 At least one of a(n), a(n)+2, a(n)+4 and a(n)+6 must be a power of 3. See comments in A264734. %e A269721 5 is a term because 5, 7, 11 are prime numbers and 9 = 3^2. %e A269721 23 is a term because 23 and 29 are prime numbers and 25 = 5^2, 27 = 3^3. %e A269721 25 is a term because 25 = 5^2, 27 = 3^3, 29 and 31 are prime numbers. %t A269721 Select[Range[0, 10^5], AllTrue[Range[0, 6, 2] + #, Or[# == 1, PrimePowerQ@ #] &] &] (* _Michael De Vlieger_, Mar 04 2016, Version 10 *) %o A269721 (PARI) lista(nn) = for(n=1, nn, if(n==1 || (isprimepower(n) && isprimepower(n+2) && isprimepower(n+4) && isprimepower(n+6)), print1(n, ", "))); %Y A269721 Cf. A000961, A006549, A120431, A264734. %K A269721 nonn,more %O A269721 1,2 %A A269721 _Altug Alkan_, Mar 04 2016