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 A077550 #6 Mar 31 2012 10:32:24 %S A077550 1,1,2,3,9,21,54,131 %N A077550 Number of nonisomorphic ways a river (or undirected line) can cross two perpendicular roads n times (orbits of A076875 under symmetry group of order 8). %C A077550 There is no constraint on touching any particular sector. %Y A077550 Cf. A076875 (total number of solutions), A076906, A076907 (if line is directed) %K A077550 nonn %O A077550 0,3 %A A077550 _N. J. A. Sloane_ and _Jon Wild_, Aug 25 2003