Half the number of 0..2 arrays of length n+2 with second differences nonzero.

Terms

a(0) =11a(1) =27a(2) =66a(3) =162a(4) =397a(5) =973a(6) =2385a(7) =5846a(8) =14329a(9) =35122a(10) =86088a(11) =211011a(12) =517211a(13) =1267741a(14) =3107372a(15) =7616509a(16) =18668898a(17) =45759514a(18) =112161581a(19) =274920321a(20) =673859821a(21) =1651704234a(22) =4048508001a(23) =9923336574a(24) =24323184920a(25) =59618790539a(26) =146132186103