OFFSET
3,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 3..1000
FORMULA
EXAMPLE
a(6) = 5 because there are 10 compositions of 6 into exactly two different parts with equal multiplicities: [1,5], [5,1], [2,4], [4,2], [1,1,2,2], [1,2,1,2], [1,2,2,1], [2,1,1,2], [2,1,2,1], [2,2,1,1].
MAPLE
a:= n-> add(iquo(d-1, 2)*binomial(2*n/d, n/d),
d=numtheory[divisors](n))/2:
seq(a(n), n=3..60);
MATHEMATICA
a[n_] := DivisorSum[n, Quotient[#-1, 2]*Binomial[2n/#, n/#]&]/2; Table[ a[n], {n, 3, 60}] (* Jean-François Alcover, Feb 28 2017, translated from Maple *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 26 2014
STATUS
approved