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 A233780 #21 Jul 21 2017 00:46:40 %S A233780 0,1,3,5,9,11,15,21,29,31,35,41,51,61,69,83,99,101,105,111,121,131, %T A233780 141,159,183,201,209,223,245,271,287,317,349,351,355,361,371,381,391, %U A233780 409,433,451,461,479,509,547,573,615,667,701,709,723,745,771 %N A233780 Number of toothpicks and D-toothpicks after n-th stage in a D-toothpick "wide" triangle (see Comments lines for definition). %C A233780 The D-toothpicks placed in northwest or northeast direction both are prohibited, except in the substructures in which the symmetry could be broken, so a(44) = 509, not 507. For another version with broken symmetry in some substructures see A233764. See also A231348, a simpler cellular automaton based in triangles which has essentially a similar structure. %C A233780 A233781 (the first differences) gives the number of toothpicks or D-toothpicks added at n-th stage. %C A233780 First differs from A169780 at a(24). %C A233780 First differs from A233970 at a(25). %C A233780 First differs from A233764 at a(44). %H A233780 N. J. A. Sloane, <a href="/wiki/Catalog_of_Toothpick_and_CA_Sequences_in_OEIS">Catalog of Toothpick and Cellular Automata Sequences in the OEIS</a> %H A233780 <a href="/index/To#toothpick">Index entries for sequences related to toothpick sequences</a> %Y A233780 Cf. A139250, A169780, A220500, A231348, A233764, A233781, A233970. %K A233780 nonn %O A233780 0,3 %A A233780 _Omar E. Pol_, Dec 15 2013