a(n) = floor(e^(n+1) - e^n).

Terms

a(0) =1a(1) =4a(2) =12a(3) =34a(4) =93a(5) =255a(6) =693a(7) =1884a(8) =5122a(9) =13923a(10) =37847a(11) =102880a(12) =279658a(13) =760190a(14) =2066413a(15) =5617093a(16) =15268842a(17) =41505016a(18) =112822331a(19) =306682894a(20) =833650539a(21) =2266097111a(22) =6159890600a(23) =16744318683a(24) =45515777207a(25) =123724710091a(26) =336318631172a(27) =914208823689