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 A309209 #20 Aug 30 2019 21:49:28
%S A309209 -1,2,36,1,40,1,94,1,1
%N A309209 Continued fraction expansion of the negation of the constant defined by A265011 (among other formulas, this is Sum_{k >= 1} arctan((-1)^k/k)).
%H A309209 W. M. Gosper, <a href="/A309204/a309204.pdf">Material from Bill Gosper's Computers & Math talk, M.I.T., 1989</a>, i+38+1 pages, annotated and scanned, included with the author's permission. (There are many blank pages because about half of the original pages were two-sided, half were one-sided.) See page 8.
%Y A309209 Cf. A265011.
%K A309209 sign,cofr,more
%O A309209 0,2
%A A309209 _N. J. A. Sloane_, Jul 29 2019; definition corrected Jul 30 2019 (Thanks to _Jianing Song_ for pointing out that something was wrong.)
%E A309209 Name corrected by _Jianing Song_, Aug 30 2019