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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
OFFSET
1,2
LINKS
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
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Oct 28 2015
EXTENSIONS
More terms from Gheorghe Coserea, May 04 2016
STATUS
approved