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 A282600 #15 Dec 19 2019 10:59:24 %S A282600 0,0,1,1,4,2,9,6,12,8,25,10,36,18,28,28,64,24,81,36,60,50,121,44,120, %T A282600 72,117,78,196,56,225,120,160,128,204,102,324,162,228,152,400,120,441, %U A282600 210,264,242,529,184,504,240,400,300,676,234,540,324,504,392,841,232 %N A282600 a(n) = Sum_(k=1..phi(n)) floor(d_k/2) where d_k are the totatives of n. %C A282600 The totatives of n are the numbers k <= n with gcd(k,n) = 1. %H A282600 Robert Israel, <a href="/A282600/b282600.txt">Table of n, a(n) for n = 1..10000</a> %H A282600 David Zmiaikou, <a href="https://tel.archives-ouvertes.fr/tel-00648120/">Origamis and permutation groups</a>, Thesis, 2011. See p. 65. %F A282600 If n is odd, a(n) = A023896(n)/2 - A000010(n)/4; %F A282600 If n is even, a(n) = A023896(n)/2 - A000010(n)/2. %o A282600 (PARI) a(n) = sum(k=1, n, (k\2)*(gcd(n, k)==1)); %Y A282600 Cf. A000010, A023896, A038566, A066840, A282601. %K A282600 nonn,look %O A282600 1,5 %A A282600 _Michel Marcus_, Feb 19 2017