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 A000206 Even sequences with period 2n. (Formerly M2372 N0940) 2
 1, 1, 3, 4, 12, 22, 71, 181, 618, 1957, 6966, 24367, 89010, 324766, 1204815, 4482400, 16802826, 63195016, 238711285, 904338163, 3436380192, 13089961012, 49979421837, 191221556269, 733014218506, 2814758323498, 10825986453978, 41700030726757, 160842946895004 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS "Even" orbits of binary necklaces of length 2n under group D_n X S_2. REFERENCES E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois J. Math., 5 (1961), 657-665. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Alois P. Heinz, Table of n, a(n) for n = 0..1000 N. J. A. Sloane, Maple code for this and related sequences FORMULA 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. - Randall L. Rathbun, Jan 11 2002 MAPLE with(numtheory): b:= proc(n) option remember;       `if`(n=0, 1, 2^(floor(n/2)-1)            +add(phi(2*d) *2^(n/d), d=divisors(n))/(4*n))     end: a:= n-> `if`(n=0, 1, `if`(irem(n, 2)=0,          (b(2*n) +b(n) +4^(n/2-1) -2^(n/2-1))/2, b(2*n)/2)): seq(a(n), n=0..30);  # Alois P. Heinz, Mar 25 2012 MATHEMATICA a[0] = 1; a11[n_] := Fold[#1 + EulerPhi[2*#2]*(2^(n/#2)/(2*n)) & , 2^Floor[n/2], Divisors[n]]/2; a[(n_)?EvenQ] := (a11[2*n] + a11[n] + 4^(n/2 - 1) - 2^(n/2 - 1))/2; a[(n_)?OddQ] := a11[2*n]/2; Table[a[n], {n, 0, 26}] (* Jean-François Alcover, Sep 01 2011, after PARI prog. *) PROG (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))} \\ Randall L. Rathbun, Jan 11 2002 CROSSREFS Cf. A000011, A000013, A000208. Sequence in context: A129922 A005221 A243391 * A240737 A075223 A071332 Adjacent sequences:  A000203 A000204 A000205 * A000207 A000208 A000209 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from Randall L. Rathbun, Jan 11 2002 STATUS approved

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Last modified June 29 01:46 EDT 2017. Contains 288856 sequences.