Number of subsets of {1..n} with two elements (possibly the same) summing to n.

Terms

a(0) =0a(1) =0a(2) =2a(3) =2a(4) =10a(5) =14a(6) =46a(7) =74a(8) =202a(9) =350a(10) =862a(11) =1562a(12) =3610a(13) =6734a(14) =14926a(15) =28394a(16) =61162a(17) =117950a(18) =249022a(19) =484922a(20) =1009210a(21) =1979054a(22) =4076206a(23) =8034314a(24) =16422922a(25) =32491550a(26) =66045982a(27) =131029082a(28) =265246810a(29) =527304974