%I A000206 M2372 N0940
%S A000206 1,1,3,4,12,22,71,181,618,1957,6966,24367,89010,324766,1204815,4482400,
%T A000206 16802826,63195016,238711285,904338163,3436380192,13089961012,
%U A000206 49979421837,191221556269,733014218506,2814758323498,10825986453978
%N A000206 Even sequences with period 2n.
%C A000206 "Even" orbits of binary necklaces of length 2n under group D_n X S_2.
%D A000206 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A000206 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A000206 E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois 
               J. Math., 5 (1961), 657-665.
%H A000206 N. J. A. Sloane, <a href="a000013.txt">Maple code for this and related 
               sequences</a>
%F A000206 a(0)=1, a(n)= (A000011(2*n)+A000011(n)+4^(n/2-1)-2^(n/2-1))/2 if n even, 
               a(n)= A000011(2*n)/2 if n odd
%o A000206 (PARI) {A000206(n)=if(n==0,1, if(n%2==0,(A000011(2*n)+A000011(n)+4^(n/
               2-1)-2^(n/2-1))/2, A000011(2*n)/2))}
%Y A000206 Cf. A000011, A000013, A000208.
%Y A000206 Sequence in context: A075221 A129922 A005221 this_sequence A075223 A071332 
               A006791
%Y A000206 Adjacent sequences: A000203 A000204 A000205 this_sequence A000207 A000208 
               A000209
%K A000206 nonn,easy,nice
%O A000206 0,3
%A A000206 N. J. A. Sloane (njas(AT)research.att.com).
%E A000206 More terms, PARI program and formula from Randall L. Rathbun, Jan 11 
               2002