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

Terms

a(0) =1a(1) =4a(2) =10a(3) =22a(4) =45a(5) =89a(6) =172a(7) =328a(8) =622a(9) =1176a(10) =2220a(11) =4186a(12) =7888a(13) =14859a(14) =27987a(15) =52710a(16) =99268a(17) =186946a(18) =352062a(19) =663010a(20) =1248588a(21) =2351350a(22) =4428075a(23) =8338971a(24) =15703984a(25) =29573802a(26) =55693492a(27) =104882184a(28) =197514502a(29) =371960008