Number of compositions of n if only the order of parts 1 and 2 matters.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =7a(5) =12a(6) =21a(7) =35a(8) =59a(9) =98a(10) =162a(11) =266a(12) =437a(13) =713a(14) =1163a(15) =1893a(16) =3077a(17) =4995a(18) =8105a(19) =13139a(20) =21293a(21) =34492a(22) =55858a(23) =90438a(24) =146406a(25) =236974a(26) =383538a(27) =620703a(28) =1004471a(29) =1625447