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 A019311 #23 Apr 19 2022 17:47:22
%S A019311 0,0,2,2,6,12,28,48,106,198,414,800,1644,3236,6546,12982,26130,52048,
%T A019311 104404,208372,417390,833930,1669102,3336476,6675512,13347600,
%U A019311 26700226,53393562,106797302,213580904,427181968,854336432,1708713470,3417372070,6834824970
%N A019311 Number of words of length n (n >= 1) over a two-letter alphabet having a minimal period of size n-2.
%H A019311 H. Harborth, <a href="http://gdz.sub.uni-goettingen.de/dms/resolveppn/?PPN=GDZPPN002189852">Endliche 0-1-Folgen mit gleichen Teilblöcken</a>, Journal für Mathematik, 271 (1974) 139-154.
%H A019311 Sean A. Irvine, <a href="https://github.com/archmageirvine/joeis/blob/master/src/irvine/oeis/a019/A019311.java">Java program</a> (github)
%e A019311 a(5) = 6 because we have: {0, 0, 1, 0, 0}, {1, 1, 0, 1, 1}, {0, 1, 0, 0, 1},
%e A019311 {0, 1, 1, 0, 1}, {1, 0, 0, 1, 0}, {1, 0, 1, 1, 0}. The first two words have autocorrelation polynomial equal to 1 + z^3 + z^4, the last four words have autocorrelation polynomial equal to 1 + z^4. - _Geoffrey Critzer_, Apr 13 2022
%Y A019311 Cf. A003000, A019310.
%K A019311 nonn
%O A019311 1,3
%A A019311 _Jeffrey Shallit_
%E A019311 More terms from _Jeffrey Shallit_, Feb 20 2013
%E A019311 More terms from _Sean A. Irvine_, Jun 20 2021