a(n) is the Wiener index of a sling on n+1 vertices.

Terms

a(0) =2a(1) =2a(2) =4a(3) =9a(4) =18a(5) =32a(6) =52a(7) =79a(8) =114a(9) =158a(10) =212a(11) =277a(12) =354a(13) =444a(14) =548a(15) =667a(16) =802a(17) =954a(18) =1124a(19) =1313a(20) =1522a(21) =1752a(22) =2004a(23) =2279a(24) =2578a(25) =2902a(26) =3252a(27) =3629a(28) =4034a(29) =4468