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 A000117 Number of even sequences with period 2n (bisection of A000011). (Formerly M1150 N0438) 3
 1, 2, 4, 8, 18, 44, 122, 362, 1162, 3914, 13648, 48734, 176906, 649532, 2405236, 8964800, 33588234, 126390032, 477353376, 1808676326, 6872485104, 26179922024, 99957747388, 382443112538, 1466024067850, 5629516646996, 21651955485304, 83400061453514 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 REFERENCES 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..500 E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois J. Math., 5 (1961), 657-665. FORMULA a(n) ~ 4^(n-1) / (2*n). - Cedric Lorand, Apr 18 2022 MATHEMATICA b[0] = 1; b[n_] := (2^Floor[n/2] + (Table[EulerPhi[2d]*2^(n/d)/(2n), {d, Divisors[n]}] // Accumulate // Last))/2; a[n_] := b[2n]; Table[a[n], {n, 0, 30}] (* Jean-François Alcover, Mar 07 2014 *) CROSSREFS Cf. A000011. Sequence in context: A006786 A125809 A245176 * A338729 A308030 A252653 Adjacent sequences: A000114 A000115 A000116 * A000118 A000119 A000120 KEYWORD nonn,easy,nice AUTHOR N. J. A. Sloane EXTENSIONS More terms from David W. Wilson, Jan 13 2000 STATUS approved

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Last modified September 22 05:50 EDT 2023. Contains 365519 sequences. (Running on oeis4.)