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 A064287 #9 Dec 22 2021 00:13:36
%S A064287 0,0,1,0,0,0,1,0,0,2,1,0,0,0,3,0,2,0,1,0,0,2,1,2,2,0,1,0,0,0,5,0,0,2,
%T A064287 1,4,2,2,1,0,0,4,1,4,4,6,1,0,0,0,1,6,0,0,1,0,2,4,3,0,2,0,9,0,0,4,3,0,
%U A064287 0,2,1,0,4,0,5,4,6,0,1,4,0,4,1,0,0,0,3,0,0,0,1,0,2,8,1,0,6,0,1,0,2,0,3,0,0
%N A064287 Number of cyclotomic cosets C of 2 mod 2n+1 such that -C is not equal to C, divided by 2.
%D A064287 F. J. MacWilliams and N. J. A. Sloane, The Theory of Error-Correcting Codes, Elsevier/North Holland, 1977, pp. 104-105.
%H A064287 Ray Chandler, <a href="/A064287/b064287.txt">Table of n, a(n) for n = 1..10000</a>
%e A064287 Mod 15 there are 4 cosets: {1, 2, 4, 8}, {3, 6, 12, 9}, {5, 10}, {7, 14, 13, 11}. Only the cosets {1, 2, 4, 8} and {7, 14, 13, 11} have the desired property, so a(7) = 2/2 = 1.
%Y A064287 A006694(n) = A064286(n) + 2*a(n).
%K A064287 nonn,nice,easy
%O A064287 1,10
%A A064287 _N. J. A. Sloane_, Sep 25 2001
%E A064287 Extended by _Ray Chandler_, Apr 25 2008