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 A282778 #16 Feb 28 2017 16:12:36 %S A282778 0,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,0,0,1,-1,1,0,2,-1,1,-1,1,0,2,-1,2,-2, %T A282778 1,-1,2,-1,2,-3,1,1,1,-1,3,-2,-1,1,0,-1,3,-1,0,0,2,-1,4,-3,0,0,1,-2,4, %U A282778 -4,3,-2,3,-3,4,-2,1,-1,2,-3,5,-3,2,-3,3,-2,3,-2,0,1 %N A282778 First differences of A281687. %C A282778 Numbers n such that a(n) = 0 are 1, 2, 4, 6, 7, 8, 10, 12, 13, 14, 16, 17, 18, 22, 28, 47, 51, 52, 57, 58, 81, 111, 112, 195, 201 according to the b-file. Can there be other values of n such that a(n) = 0? See also graph of this sequence. %H A282778 Altug Alkan, <a href="/A282778/b282778.txt">Table of n, a(n) for n = 1..10000</a> %H A282778 Altug Alkan, <a href="/A282778/a282778.png">Alternative Scatterplot of A282778</a> %F A282778 a(n) = A281687(n+1) - A281687(n). %o A282778 (PARI) a281687(n) = sum(k=1, n, istotient(k) && istotient(2*n-k)); %o A282778 a(n) = a281687(n+1) - a281687(n); %Y A282778 Cf. A281687. %K A282778 sign %O A282778 1,23 %A A282778 _Altug Alkan_, Feb 21 2017