Number of subsets of {1..n} containing two distinct elements summing to n.

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =4a(5) =14a(6) =28a(7) =74a(8) =148a(9) =350a(10) =700a(11) =1562a(12) =3124a(13) =6734a(14) =13468a(15) =28394a(16) =56788a(17) =117950a(18) =235900a(19) =484922a(20) =969844a(21) =1979054a(22) =3958108a(23) =8034314a(24) =16068628a(25) =32491550a(26) =64983100a(27) =131029082a(28) =262058164a(29) =527304974