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 A157001 #3 Jun 26 2025 11:46:41 %S A157001 0,1,2,5,7,11,14,21,25,32,37,47,53,63,70,85,93,106,115,132,142,158, %T A157001 169,191,203,222,235,259,273,295,310,341,357,382,399,430,448,476,495, %U A157001 532,552,583,604,642,664,698,721,767,791,828,853,898,924,964,991,1043,1071 %N A157001 Fractions x/y, with 1<=x, y<=n, that reduce to (odd)/(even). %C A157001 Equivalently, a(n) is the number of fractions x/y, with 1<=x, y<=n, that reduce to (even)/(odd). %Y A157001 a(n) = (n^2-A135013)/2. %K A157001 nonn %O A157001 1,3 %A A157001 Adam McDougall (mcdougall.adam(AT)gmail.com), Feb 20 2009