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%I #17 Oct 10 2018 03:24:37
%S 1,7,9,29,19,63,33,107,74,133,73,285,99,231,219,393,163,518,201,623,
%T 393,511,289,1155,422,693,634,1101,451,1533,513,1479,897,1141,915,
%U 2482,723,1407,1227,2609,883,2751,969,2477,2078,2023,1153,4569
%N The number c_{Z3 pi_1(B_1)}(2n) of 3-torus 2n-coverings over the first amphicosm.
%H Gheorghe Coserea, <a href="/A263826/b263826.txt">Table of n, a(n) for n = 1..20000</a>
%H G. Chelnokov, M. Deryagina, A. Mednykh, <a href="http://arxiv.org/abs/1502.01528">On the Coverings of Amphicosms; Revised title: On the coverings of Euclidian manifolds B_1 and B_2</a>, arXiv preprint arXiv:1502.01528 [math.AT], 2015.
%t a[n_] := 1/2 Sum[Sum[(d^2 + 5/2 + 3/2 (-1)^Mod[d, 2]) m, {m, Divisors[n/d]} ], {d, Divisors[n]}];
%t Array[a, 48] (* _Jean-François Alcover_, Oct 10 2018, after _Gheorghe Coserea_ *)
%o (PARI)
%o a(n) = 1/2 * sumdiv(n, d, sumdiv(n\d, m, (d^2 + 5/2 + 3/2*(-1)^(d%2))*m));
%o vector(48, n, a(n)) \\ _Gheorghe Coserea_, May 04 2016
%Y Cf. A263825-A263830, A263832.
%K nonn
%O 1,2
%A _N. J. A. Sloane_, Oct 28 2015
%E More terms from _Gheorghe Coserea_, May 04 2016
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