First differences of A003946.

Terms

a(0) =1a(1) =3a(2) =8a(3) =24a(4) =72a(5) =216a(6) =648a(7) =1944a(8) =5832a(9) =17496a(10) =52488a(11) =157464a(12) =472392a(13) =1417176a(14) =4251528a(15) =12754584a(16) =38263752a(17) =114791256a(18) =344373768a(19) =1033121304a(20) =3099363912a(21) =9298091736a(22) =27894275208a(23) =83682825624a(24) =251048476872a(25) =753145430616