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

Terms

a(0) =1a(1) =4a(2) =10a(3) =22a(4) =46a(5) =94a(6) =188a(7) =372a(8) =734a(9) =1446a(10) =2844a(11) =5588a(12) =10976a(13) =21556a(14) =42330a(15) =83120a(16) =163214a(17) =320484a(18) =629292a(19) =1235652a(20) =2426272a(21) =4764118a(22) =9354602a(23) =18368260a(24) =36067056a(25) =70819582a(26) =139058010a(27) =273047782a(28) =536143806a(29) =1052746804