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A263828 The number c_{P c pi_1(B_1)}(n) of the first amphicosm n-coverings over the first amphicosm. 2
1, 4, 5, 10, 7, 20, 9, 22, 18, 28, 13, 50, 15, 36, 35, 46, 19, 72, 21, 70, 45, 52, 25, 110, 38, 60, 58, 90, 31, 140, 33, 94, 65, 76, 63, 180, 39, 84, 75, 154, 43, 180, 45, 130, 126, 100, 49, 230, 66, 152, 95, 150, 55, 232, 91, 198, 105, 124, 61 (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_] := Sum[(3/2 + 1/2 (-1)^Mod[d, 2]) DivisorSigma[1, n/d], {d, Divisors[ n]}] - If[OddQ[n], 0, Sum[(3/2 + 1/2 (-1)^Mod[d, 2]) DivisorSigma[1, n/(2 d)], {d, Divisors[n/2]}]];

Array[a, 59] (* Jean-Fran├žois Alcover, Oct 10 2018, after Gheorghe Coserea *)

PROG

(PARI)

a(n) = {

  sumdiv(n, d, (3/2 + 1/2*(-1)^(d%2)) * sigma(n/d)) -

  if (n%2, 0, sumdiv(n\2, d, (3/2 + 1/2*(-1)^(d%2))*sigma(n\(2*d))))

};

vector(59, n, a(n))  \\ Gheorghe Coserea, May 04 2016

CROSSREFS

Cf. A263825-A263830, A263832.

Sequence in context: A215754 A226486 A060648 * A327577 A092961 A177711

Adjacent sequences:  A263825 A263826 A263827 * A263829 A263830 A263831

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 September 21 23:55 EDT 2019. Contains 327286 sequences. (Running on oeis4.)