Number of 2-compositions of n having no increasing columns.

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =18a(5) =44a(6) =110a(7) =272a(8) =676a(9) =1676a(10) =4160a(11) =10320a(12) =25608a(13) =63536a(14) =157648a(15) =391152a(16) =970528a(17) =2408064a(18) =5974880a(19) =14824832a(20) =36783296a(21) =91266496a(22) =226449920a(23) =561866240a(24) =1394099328a(25) =3459031296a(26) =8582528768a(27) =21294921472a(28) =52836837888a(29) =131098461184