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 A263826 The number c_{Z3 pi_1(B_1)}(2n) of 3-torus 2n-coverings over the first amphicosm. 2
 1, 7, 9, 29, 19, 63, 33, 107, 74, 133, 73, 285, 99, 231, 219, 393, 163, 518, 201, 623, 393, 511, 289, 1155, 422, 693, 634, 1101, 451, 1533, 513, 1479, 897, 1141, 915, 2482, 723, 1407, 1227, 2609, 883, 2751, 969, 2477, 2078, 2023, 1153, 4569 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Gheorghe Coserea, Table of n, a(n) for n = 1..20000 G. Chelnokov, M. Deryagina, A. Mednykh, On the Coverings of Amphicosms; Revised title: On the coverings of Euclidian manifolds B_1 and B_2, arXiv preprint arXiv:1502.01528 [math.AT], 2015. MATHEMATICA a[n_] := 1/2 Sum[Sum[(d^2 + 5/2 + 3/2 (-1)^Mod[d, 2]) m, {m, Divisors[n/d]} ], {d, Divisors[n]}]; Array[a, 48] (* Jean-François Alcover, Oct 10 2018, after Gheorghe Coserea *) PROG (PARI) a(n) = 1/2 * sumdiv(n, d, sumdiv(n\d, m, (d^2 + 5/2 + 3/2*(-1)^(d%2))*m)); vector(48, n, a(n))  \\ Gheorghe Coserea, May 04 2016 CROSSREFS Cf. A263825-A263830, A263832. Sequence in context: A324986 A030404 A242505 * A066930 A247192 A154260 Adjacent sequences:  A263823 A263824 A263825 * A263827 A263828 A263829 KEYWORD nonn AUTHOR N. J. A. Sloane, Oct 28 2015 EXTENSIONS More terms from Gheorghe Coserea, May 04 2016 STATUS approved

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Last modified August 12 23:52 EDT 2022. Contains 356077 sequences. (Running on oeis4.)