Analog of Motzkin numbers for Coxeter type D.

Terms

a(0) =1a(1) =4a(2) =11a(3) =31a(4) =87a(5) =246a(6) =699a(7) =1996a(8) =5723a(9) =16468a(10) =47533a(11) =137567a(12) =399073a(13) =1160082a(14) =3378483a(15) =9855207a(16) =28790403a(17) =84218052a(18) =246651729a(19) =723165765a(20) =2122391109a(21) =6234634266a(22) =18330019029a(23) =53932825926a(24) =158802303429a(25) =467898288676