A263826 The number c_{Z3 pi_1(B_1)}(2n) of 3-torus 2n-coverings over the first amphicosm.

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

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