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 A260185 #36 Oct 27 2023 20:49:18
%S A260185 1,3,9,21,63,111,333,693,1521,2577,7731,13491,40473,67833,119241,
%T A260185 239481,718443,1340523,4021569,7494849,13356657,22271409,66814227,
%U A260185 130266387,268286823,447212583,896472063,1684872063,5054616189,9566769789,28700309367,57402497367
%N A260185 a(n) is the number of ways to select an ordered pair of subsets of {2,...,n} such that each pair of elements from different subsets are relatively prime.
%C A260185 This sequence was used by LuoYuping when he set a problem for NOI 2015 Day1 Problem3.
%C A260185 a(n) is the number of ways to find X and Y where set X and Y are subsets of {2,...,n}, and for all a in X and all b in Y, gcd(a,b) = 1. Also note that X or Y can be empty.
%D A260185 National Olympiad in Informatics 2015, China, Day 1 Problem 3.
%H A260185 Alois P. Heinz, <a href="/A260185/b260185.txt">Table of n, a(n) for n = 1..100</a> (first 80 terms from Giovanni Resta)
%H A260185 Sirius Caffrey, <a href="/A260185/a260185_2.txt">C++ program for A260185</a>
%H A260185 Sirius Caffrey, <a href="/A260185/a260185_1.txt">Python program for A260185</a>
%F A260185 a(p) = 3*a(p-1) for p prime. - _Alois P. Heinz_, Jul 19 2015
%e A260185 For n=1 the 1 pair of sets is [{},{}].
%e A260185 For n=2 the 3 pairs of sets are [{},{}], [{2},{}], and [{},{2}].
%e A260185 For n=3 the 9 pairs of sets are [{},{}], [{2},{}], [{},{2}], [{3},{}], [{},{3}], [{2,3},{}], [{},{2,3}], [{2},{3}], and [{3},{2}].
%o A260185 (C++) // see link above
%o A260185 (Python) # see link above
%K A260185 nonn
%O A260185 1,2
%A A260185 _Sirius Caffrey_, Jul 17 2015
%E A260185 a(31)-a(32) from _Giovanni Resta_, Jul 18 2015