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 A056139 #7 Mar 20 2018 19:02:15 %S A056139 0,0,1,0,1,0,-13,4,23,0,1,0,-25,4,35,0,1,0,-37,4,47,0,-91,-42,9,62, %T A056139 117,0,1,0,-123,-58,9,78,149,0,-73,4,83,0,1,0,-85,4,95,0,-187,-90,9, %U A056139 110,213,0,-211,-102,9,122,237,0,1,0,-243,-118,9,138,269,0,-133,4,143,0,1,0,-291,-142,9,162,317,0,-157,4,167,0,-331,-162,9 %N A056139 a(n) = n^2 - primefloor(n)*primeceiling(n). %C A056139 a(n)= 0 iff n is prime. %H A056139 Antti Karttunen, <a href="/A056139/b056139.txt">Table of n, a(n) for n = 2..65537</a> %F A056139 a(n) = n^2 - A007917(n)*A007918(n) = A000290(n) - A030664(n). %e A056139 a(3)=3^2-3*3=0, a(4)=4^2-3*5=1 %o A056139 (PARI) %o A056139 A030664(n) = if (n < 2, 1, precprime(n)*nextprime(n)); \\ From A030664 %o A056139 A056139(n) = (n^2 - A030664(n)); \\ _Antti Karttunen_, Mar 20 2018 %Y A056139 Cf. A007917, A007918, A030664, A056140, A056141. %K A056139 easy,sign %O A056139 2,7 %A A056139 _Henry Bottomley_, Jun 15 2000 %E A056139 More terms from _Antti Karttunen_, Mar 20 2018