Number of trapezoidal words of length n.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =16a(5) =28a(6) =46a(7) =70a(8) =102a(9) =140a(10) =190a(11) =250a(12) =318a(13) =398a(14) =496a(15) =602a(16) =724a(17) =862a(18) =1018a(19) =1192a(20) =1382a(21) =1584a(22) =1816a(23) =2070a(24) =2340a(25) =2630a(26) =2956a(27) =3300a(28) =3668a(29) =4064