OFFSET
1,6
LINKS
Harry J. Smith, Table of n, a(n) for n = 1..200
EXAMPLE
n=6, binomial(6,3) = 20 = 4*5, D = {1,2,4,5,10,20} of which the non-unitary divisors are 2 and 10 with Sum = a(6) = 12.
MATHEMATICA
nus[n_] := If[n==1, 0, DivisorSigma[1, n] - Times @@ (1 + Power @@@ FactorInteger[n])]; Table[nus@ Binomial[n, Floor[n/2]], {n, 35}] (* Giovanni Resta, Jun 22 2018 *)
PROG
(PARI) usigma(n)= { my(f, s=1); f=factor(n); for(i=1, matsize(f)[1], s*=1 + f[i, 1]^f[i, 2]); return(s) }
a(n)={my(b=binomial(n, n\2)); sigma(b) - usigma(b); } \\ Harry J. Smith, Sep 08 2009
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Sep 11 2001
STATUS
approved