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 A353028 #10 Jun 11 2022 13:48:37 %S A353028 0,1,2,5,15,47,156,527,1826,6407,22762,81587,294711,1070973,3912808, %T A353028 14358493,52893647,195498831,724704974,2693397035,10033263089, %U A353028 37452388361,140063288728 %N A353028 Number of subgraphs of the directed square lattice with n vertices and all vertices reachable from the origin. %C A353028 Similar to A344571 but here n is the number of vertices. %F A353028 a(n) >= 2*a(n-1) for n > 0. %e A353028 In the following examples, the origin is in the bottom left corner and graph edges are directed upwards and to the right. %e A353028 The a(1) = 1 graph is: %e A353028 . %e A353028 The a(2) = 2 graphs are: %e A353028 | __ %e A353028 . %e A353028 The a(3) = 5 graphs are: %e A353028 | __ %e A353028 | | __.__ __| |__ %e A353028 . %e A353028 The a(4) = 15 graphs are: %e A353028 | __ %e A353028 | | |__ __| __.__ | __ %e A353028 | | | | | |__ |__ %e A353028 . %e A353028 __ | __ %e A353028 __.__.__ __.__| __|__ __| __| |__.__ |__| |__| %e A353028 . %e A353028 Other examples with 5, 6, and 7 vertices respectively include: %e A353028 | __.__ __|__| %e A353028 |__| |__.__| |__| %Y A353028 Cf. A344571. %K A353028 nonn,more %O A353028 0,3 %A A353028 _Roman Hros_, Apr 18 2022