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 A078105 #7 Mar 31 2012 10:32:24 %S A078105 1,0,1,1,2,1,8,8,48,54,331,439,2558,3734,21057,33384,182293,307719, %T A078105 1638465,2913775,15181584,28194412,144206012,277887666,1398566992 %N A078105 Number of nonisomorphic ways a loop can cross three roads meeting in a Y n times (orbits under symmetry group of order 6). %C A078105 There is no constraint on touching any particular sector. %C A078105 The Mercedes-Benz problem: closed meanders crossing a Y. %H A078105 Anonymous, <a href="/A078105/a078105.a.gif">Illustration for a(3) = 1</a> %e A078105 With three crossings the loop must cut each road exactly once, so a(3) = 1. %e A078105 With 4 crossings the loop can cut one road 4 times (one possibility), or two roads twice each (one possibility), so a(4) = 2. %Y A078105 Cf. A078104 (total number of solutions), A077460 and A005315 (loop crossing one road). %K A078105 nonn,nice %O A078105 0,5 %A A078105 _N. J. A. Sloane_ and _Jon Wild_, Dec 05 2002