Number of transitive relations on an n-set with exactly four ordered pairs.

Terms

a(0) =0a(1) =0a(2) =1a(3) =45a(4) =549a(5) =3755a(6) =18120a(7) =69006a(8) =220710a(9) =616554a(10) =1545435a(11) =3544915a(12) =7552611a(13) =15119325a(14) =28699034a(15) =52032540a(16) =90643260a(17) =152465316a(18) =248625765a(19) =394404489a(20) =610396945a(21) =923906655a(22) =1370595996a(23) =1996425530a(24) =2859913794a(25) =4034751150a(26) =5612802975a(27) =7707539151a(28) =10457928495