A Pell-related fourth-order recurrence.

Terms

a(0) =0a(1) =1a(2) =4a(3) =19a(4) =88a(5) =401a(6) =1804a(7) =8051a(8) =35760a(9) =158401a(10) =700564a(11) =3095731a(12) =13673224a(13) =60375953a(14) =266559388a(15) =1176763859a(16) =5194762080a(17) =22931453953a(18) =101225940772a(19) =446836798675