a(n) = number of components of the graph P(n,2) (defined in Comments).

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =1a(5) =2a(6) =4a(7) =11a(8) =13a(9) =19a(10) =29a(11) =43a(12) =107a(13) =169a(14) =350a(15) =603a(16) =1134a(17) =2070a(18) =3803a(19) =7502a(20) =13989a(21) =26495a(22) =50826a(23) =97369a(24) =185827a(25) =357307a(26) =690577a(27) =1332382a(28) =2565110a(29) =4958962