a(1) = b(1) = 0; for n > 1, b(n) = b(n-1) + n-1 + a(n-1) and a(n) = a(n-1) + n-1 + b(n).

Terms

a(0) =0a(1) =2a(2) =9a(3) =29a(4) =83a(5) =226a(6) =602a(7) =1588a(8) =4171a(9) =10935a(10) =28645a(11) =75012a(12) =196404a(13) =514214a(14) =1346253a(15) =3524561a(16) =9227447a(17) =24157798a(18) =63245966a(19) =165580120a(20) =433494415a(21) =1134903147a(22) =2971215049a(23) =7778742024a(24) =20365011048a(25) =53316291146a(26) =139583862417