a(n) = a(n-1) + 2*a(n-2) + 3*a(n-3), for n>3, otherwise a(n) = n.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =11a(5) =25a(6) =59a(7) =142a(8) =335a(9) =796a(10) =1892a(11) =4489a(12) =10661a(13) =25315a(14) =60104a(15) =142717a(16) =338870a(17) =804616a(18) =1910507a(19) =4536349a(20) =10771211a(21) =25575430a(22) =60726899a(23) =144191392a(24) =342371480a(25) =812934961a(26) =1930252097a(27) =4583236459a(28) =10882545536a(29) =25839774745