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 A283021 #5 Feb 28 2017 22:41:37 %S A283021 0,2,4,6,12,30,70,168,412,1014,2514,6270,15702,39468,99516,251586, %T A283021 637500 %N A283021 Erroneous version of A263656. %C A283021 Included in accordance with OEIS policy of including erroneous but published sequences to serve as pointers to the correct versions. %C A283021 Number of length-2n central circular binary strings without zigzags (see reference for precise definition). %C A283021 Differs from A263656 at only a(1). %C A283021 See page 6, "Central strings", which gives this almost certainly incorrect sequence. %C A283021 Compare with leading diagonal of Fig 1 (which is A253656) %C A283021 [The other possible rationale for this discrepanncy would be that this sequence also counts the strings 01 and 10. However these are excluded from the other sequences and formula in the same paper.] %H A283021 E. Munarini and N. Z. Salvi, <a href="http://www.emis.de/journals/INTEGERS/papers/d19/d19.Abstract.html">Circular Binary Strings without Zigzags</a>, Integers: Electronic Journal of Combinatorial Number Theory 3 (2003), #A19. %Y A283021 Cf. A263656. %K A283021 dead %O A283021 0,2 %A A283021 _Andrew Howroyd_, Feb 26 2017