a(n) = a(n-1) + a(n-2) + a(n-3), with a(1) = 4, a(2) = 13, a(3) = 42.

Terms

a(0) =4a(1) =13a(2) =42a(3) =59a(4) =114a(5) =215a(6) =388a(7) =717a(8) =1320a(9) =2425a(10) =4462a(11) =8207a(12) =15094a(13) =27763a(14) =51064a(15) =93921a(16) =172748a(17) =317733a(18) =584402a(19) =1074883a(20) =1977018a(21) =3636303a(22) =6688204a(23) =12301525a(24) =22626032a(25) =41615761a(26) =76543318a(27) =140785111a(28) =258944190a(29) =476272619