OFFSET
1,5
LINKS
Eric Weisstein's World of Mathematics, Refactorable Number
FORMULA
a(n) = Sum_{i=1..n} (1-sign(i mod d(i))) * (1-sign((2n-i) mod d(2n-i))) where d(n) is the number of divisors of n.
EXAMPLE
a(5) = 2; There are two partitions of 2*5 = 10 into two refactorable parts: (1,9) and (2,8).
MAPLE
MATHEMATICA
Table[Sum[(1 - Sign[Mod[i, DivisorSigma[0, i]]]) (1 - Sign[Mod[#, DivisorSigma[0, #]]] &[2 n - i]), {i, n}], {n, 90}] (* Michael De Vlieger, Jan 07 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, Jan 06 2017
STATUS
approved