Number of subsets of {1..n} whose cardinality is the sum of two distinct elements.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =7a(6) =17a(7) =40a(8) =90a(9) =199a(10) =435a(11) =939a(12) =2007a(13) =4258a(14) =8976a(15) =18817a(16) =39263a(17) =81595a(18) =168969a(19) =348820a(20) =718134a(21) =1474863a(22) =3022407a(23) =6181687a(24) =12621135a(25) =25727686a(26) =52369508a(27) =106460521a(28) =216162987a(29) =438431215