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%I #13 Dec 22 2021 10:18:28
%S 1,1,0,2,1,1,2,2,1,1,0,2,3,1,0,4,1,1,2,2,3,3,0,0,3,1,2,4,1,1,2,6,1,1,
%T 0,0,3,1,0,4,1,3,2,0,1,1,2,2,7,1,0,2,1,3,2,4,1,3,2,2,3,3,0,10,1,1,4,2,
%U 1,1,2,6,1,1,0,4,1,3,2,0,1,5,0,2,11,1,2,4,1,1,2,6,3,3,0,2,7,1,0,4,1,11,2,4
%N Number of cyclotomic cosets C of 2 mod 2n+1 such that -C = C.
%D F. J. MacWilliams and N. J. A. Sloane, The Theory of Error-Correcting Codes, Elsevier/North Holland, 1977, pp. 104-105.
%H Ray Chandler, <a href="/A064286/b064286.txt">Table of n, a(n) for n = 1..10000</a>
%F A006694(n) = a(n) + 2*A064287(n).
%e Mod 15 there are 4 cosets: {1, 2, 4, 8}, {3, 6, 12, 9}, {5, 10}, {7, 14, 13, 11}. Only the cosets {3, 6, 12, 9} and {5, 10} have the desired property. So a(7) = 2.
%Y Cf. A006694, A064287.
%K nonn,nice,easy
%O 1,4
%A _N. J. A. Sloane_, Sep 25 2001
%E Extended by _Ray Chandler_, Apr 25 2008
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