Poincaré series for hyperbolic reflection group with Coxeter diagram shown in Comments.

Terms

a(0) =1a(1) =4a(2) =10a(3) =22a(4) =44a(5) =84a(6) =156a(7) =284a(8) =512a(9) =918a(10) =1642a(11) =2932a(12) =5230a(13) =9324a(14) =16618a(15) =29614a(16) =52768a(17) =94020a(18) =167516a(19) =298460a(20) =531756a(21) =947406a(22) =1687946a(23) =3007324a(24) =5357986a(25) =9546028a(26) =17007626a(27) =30301534a(28) =53986540a(29) =96184780