Number of arrays of the median of three adjacent elements of some length n+2 0..2 array.

Terms

a(0) =3a(1) =9a(2) =27a(3) =77a(4) =185a(5) =447a(6) =1071a(7) =2593a(8) =6333a(9) =15513a(10) =38039a(11) =93213a(12) =228279a(13) =558849a(14) =1367945a(15) =3348489a(16) =8196847a(17) =20065943a(18) =49122291a(19) =120253773a(20) =294386457a(21) =720668887a(22) =1764221329a(23) =4318870217a(24) =10572732933a(25) =25882394413a(26) =63360952825a(27) =155109705901