Cluster series for square lattice.

Terms

a(0) =1a(1) =4a(2) =12a(3) =24a(4) =52a(5) =108a(6) =224a(7) =412a(8) =844a(9) =1528a(10) =3152a(11) =5036a(12) =11984a(13) =15040a(14) =46512a(15) =34788a(16) =197612a(17) =4036a(18) =929368a(19) =-702592a(20) =4847552a(21) =-7033956a(22) =27903296a(23) =-54403996a(24) =170579740