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 A261928 #20 Aug 07 2018 12:48:43
%S A261928 1,3,6,8,15,18,28,28,36,45,66,48,91,84,90,96,153,108,190,120,168,198,
%T A261928 276,168,275,273,297,224,435,270,496,368,396,459,420,288,703,570,546,
%U A261928 420,861,504,946,528,540,828,1128,576,1078,825,918,728,1431,891,990,784,1140,1305,1770,720,1891,1488,1008,1408,1365,1188,2278
%N A261928 a(n) is the number of different pairs (x*y,x+y) mod n.
%H A261928 Andrew Howroyd, <a href="/A261928/b261928.txt">Table of n, a(n) for n = 1..1000</a>
%F A261928 a(n) = n^2 - A261929(n).
%e A261928 a(2) = 3 because there exist only the pairs (0,0), (0,1) and (1,0) as results from (x*y,x+y) mod n. There are no x,y with (x*y,x+y)=(1,1) mod 2.
%o A261928 (PARI) a(n)={my(v=vector(n)); for(i=1, n, for(j=1, n, v[j]=bitor(v[j], 1<<(i*(j-i)%n)))); sum(j=1, n, hammingweight(v[j]))} \\ _Andrew Howroyd_, Aug 01 2018
%Y A261928 Cf. A261929 (number of other pairs).
%K A261928 nonn,mult
%O A261928 1,2
%A A261928 _Thomas Kerscher_, Sep 06 2015
%E A261928 Keyword:mult added by _Andrew Howroyd_, Aug 01 2018