Analog of Motzkin sums for Coxeter type D.

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =105a(5) =306a(6) =889a(7) =2584a(8) =7515a(9) =21880a(10) =63778a(11) =186132a(12) =543855a(13) =1590876a(14) =4658580a(15) =13655472a(16) =40065243a(17) =117654876a(18) =345786396a(19) =1017040380a(20) =2993498739a(21) =8816790906a(22) =25984489545a(23) =76625467128a(24) =226085062525a(25) =667415280376