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 A105690 #3 Mar 31 2012 13:46:51 %S A105690 2,1,2,1,1,2,1,2,1,1,1,2,2,1,1,1,1,2,2,2,1,1,1,1,1,1,2,1,2,2,2,2,1,1, %T A105690 1,1,1,1,1,1,2,1,1,2,2,1,2,2,2,1,2,1,2,1,1,1,1,1,1,1,1,2,1,1,1,1,2,1, %U A105690 2,2,1,1,2,2,2,2,1,2 %N A105690 A simple "Fractal Jump Sequence" (FJS). %C A105690 See A105397 for definition of Fractal Jump Sequence. %C A105690 This is how to construct the sequence: start with 2 on rows a and b; put 2 empty spaces behind the 2 on row a; choose any two digits and put them on row b under the 2 empty spaces of row a; go back to row a and add the same two digits but each one with its according spaces (1 must always be followed by 1 space on row a and 2 must always be followed by 2 spaces); go back to row b and add under the next available spaces of a the digits necessary so to have the same succession of digits in rows b and a. The sequence builds itself automatically. The row (c) is obtained by "pushing" (a) into (b) -- [the first digit of a and b melt in a single copy of themselves]. Row (c) is the FJS sequence above. %C A105690 (a)..2..1.2..1.1.2..1.2..1.1.1.2..2..... %C A105690 (b)..212.1.12.1.2.11.1.22.1.1.1.12.22... %C A105690 ---------------------------------------- %C A105690 (c)..21211212111221111222111111212222... %e A105690 To build such sequences one has only to choose the first digit d and the d digits to put under the d spaces of row (a). %K A105690 base,easy,nonn %O A105690 2,1 %A A105690 _Eric Angelini_, May 04 2005