a(n) = a(n-1) + a(n-2) + 3*a(n-3), with a(0) = 1, a(1) = 0, a(2) = 1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =6a(6) =11a(7) =23a(8) =52a(9) =108a(10) =229a(11) =493a(12) =1046a(13) =2226a(14) =4751a(15) =10115a(16) =21544a(17) =45912a(18) =97801a(19) =208345a(20) =443882a(21) =945630a(22) =2014547a(23) =4291823a(24) =9143260a(25) =19478724a(26) =41497453a(27) =88405957a(28) =188339582a(29) =401237898