Tetranacci numbers arising in connection with current algebras sp(2)_n.

Terms

a(0) =3a(1) =5a(2) =10a(3) =19a(4) =37a(5) =71a(6) =137a(7) =264a(8) =509a(9) =981a(10) =1891a(11) =3645a(12) =7026a(13) =13543a(14) =26105a(15) =50319a(16) =96993a(17) =186960a(18) =360377a(19) =694649a(20) =1338979a(21) =2580965a(22) =4974970a(23) =9589563a(24) =18484477a(25) =35629975a(26) =68678985a(27) =132383000a(28) =255176437a(29) =491868397