OFFSET
1,1
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.
MAPLE
A263827 := proc(n)
local locn, a, twol, fourl ;
locn := 2*n ;
# Theorem 3 (iii)
a := 0 ;
for twol in numtheory[divisors](locn) do
if type(twol, 'even') then
a := a+numtheory[sigma](locn/twol) ;
end if;
end do:
for fourl in numtheory[divisors](locn) do
if modp(fourl, 4) = 0 then
a := a-numtheory[sigma](locn/fourl) ;
end if;
end do:
%*2 ;
end proc: # R. J. Mathar, Nov 03 2015
MATHEMATICA
a[n_] := 2*Sum[If[Mod[d, 4] == 2, DivisorSigma[1, 2*n/d], 0], {d, Divisors[ 2*n ] } ];
Array[a, 70] (* Jean-François Alcover, Dec 03 2017 *)
PROG
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Oct 28 2015
STATUS
approved