Number of 2-element intersecting families whose union is an n-element set.

Terms

a(0) =0a(1) =2a(2) =9a(3) =32a(4) =105a(5) =332a(6) =1029a(7) =3152a(8) =9585a(9) =29012a(10) =87549a(11) =263672a(12) =793065a(13) =2383292a(14) =7158069a(15) =21490592a(16) =64504545a(17) =193579172a(18) =580868589a(19) =1742867912a(20) =5229128025a(21) =15688432652a(22) =47067395109a(23) =141206379632a(24) =423627527505