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 A019309 #25 Dec 01 2015 09:00:54 %S A019309 1,4,12,48,180,720,2832,11328,45132,180528,721392,2885568,11539440, %T A019309 46157760,184619712,738478848,2953870260,11815481040,47261743632, %U A019309 189046974528,756187176720,3024748706880,12098991941952 %N A019309 Number of "bifix-free" words of length n over a four-letter alphabet. %H A019309 E. Barcucci, A. Bernini, S. Bilotta, R. Pinzani, <a href="http://arxiv.org/abs/1502.05275">Cross-bifix-free sets in two dimensions</a>, arXiv preprint arXiv:1502.05275 [cs.DM], 2015. %H A019309 S. Bilotta, E. Pergola and R. Pinzani, <a href="http://arxiv.org/abs/1112.3168">A new approach to cross-bifix-free sets</a>, arXiv preprint arXiv:1112.3168 [cs.FL], 2011. %H A019309 T. Harju and D. Nowotka, <a href="http://www.tucs.fi/Publications/attachment.php?fname=TR546.pdf">Border correlation of binary words</a>. %H A019309 P. Tolstrup Nielsen, <a href="http://dx.doi.org/10.1109/TIT.1973.1055065">A note on bifix-free sequences</a>, IEEE Trans. Info. Theory IT-19 (1973), 704-706. %F A019309 a(2n+1) = 4a(2n); a(2n) = 4a(2n-1) - a(n). %t A019309 a[0]=1; a[n_]:=a[n]=4*a[n-1]-If[EvenQ[n], a[n/2], 0] (* _Ed Pegg Jr_, Jan 05 2005 *) %Y A019309 Cf. A003000, A019308, A094547, A094559, A094578. %K A019309 nonn,easy %O A019309 0,2 %A A019309 _Jeffrey Shallit_