A modular recurrence.

Terms

a(0) =1a(1) =3a(2) =15a(3) =45a(4) =225a(5) =675a(6) =3375a(7) =10125a(8) =50625a(9) =151875a(10) =759375a(11) =2278125a(12) =11390625a(13) =34171875a(14) =170859375a(15) =512578125a(16) =2562890625a(17) =7688671875a(18) =38443359375a(19) =115330078125a(20) =576650390625