Number of length-4 0..n arrays with no adjacent pair x,x+1 repeated.

Terms

a(0) =1a(1) =15a(2) =79a(3) =253a(4) =621a(5) =1291a(6) =2395a(7) =4089a(8) =6553a(9) =9991a(10) =14631a(11) =20725a(12) =28549a(13) =38403a(14) =50611a(15) =65521a(16) =83505a(17) =104959a(18) =130303a(19) =159981a(20) =194461a(21) =234235a(22) =279819a(23) =331753a(24) =390601a(25) =456951a(26) =531415a(27) =614629a(28) =707253a(29) =809971