a(n) = floor(e^(n + 1/2)).

Terms

a(0) =1a(1) =4a(2) =12a(3) =33a(4) =90a(5) =244a(6) =665a(7) =1808a(8) =4914a(9) =13359a(10) =36315a(11) =98715a(12) =268337a(13) =729416a(14) =1982759a(15) =5389698a(16) =14650719a(17) =39824784a(18) =108254987a(19) =294267566a(20) =799902177a(21) =2174359553a(22) =5910522063a(23) =16066464720a(24) =43673179097a(25) =118716009132a(26) =322703570371