OFFSET
0,4
COMMENTS
a(0) = 0 assuming tau(0) = 0. - Michael Somos, Mar 05 2014
LINKS
Robert Israel, Table of n, a(n) for n = 0..10000
FORMULA
G.f.: -1/x + (1 - x)^2*Sum_{k>=1} x^(k-2)/(1 - x^k). - Ilya Gutkovskiy, Jan 29 2017
EXAMPLE
G.f. = - x + x^2 - 2*x^3 + 3*x^4 - 4*x^5 + 4*x^6 - 3*x^7 + 2*x^8 - 3*x^9 + ...
MAPLE
Taus:= map(numtheory:-tau, [$0..100]):
Taus[3..-1] - 2*Taus[2..-2] + Taus[1..-3]; # Robert Israel, Dec 07 2015
MATHEMATICA
Join[{0}, Differences[DivisorSigma[0, Range[90]], 2]] (* Harvey P. Dale, Jan 10 2014 *)
a[ n_] := SeriesCoefficient[ (QHypergeometricPFQ[ {x, x}, {x^2, x^2}, x, x^2] - 1) / x, {x, 0, n}]; (* Michael Somos, Mar 05 2014 *)
CROSSREFS
KEYWORD
sign
AUTHOR
N. J. A. Sloane, Dec 17 1999
STATUS
approved