Partial sums of walks of length n+1 on a tetrahedron A001998.

Terms

a(0) =1a(1) =3a(2) =7a(3) =17a(4) =42a(5) =112a(6) =308a(7) =882a(8) =2563a(9) =7565a(10) =22449a(11) =66979a(12) =200204a(13) =599514a(14) =1796350a(15) =5385764a(16) =16150725a(17) =48442327a(18) =145307291a(19) =435892341a(20) =1307617966a(21) =3922765316a(22) =11768118792a(23) =35304090646a(24) =105911740487a(25) =317734424289a(26) =953201678533