Shifts 2 places left under boustrophedon transform.

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =2a(5) =6a(6) =17a(7) =62a(8) =259a(9) =1230a(10) =6592a(11) =39313a(12) =258575a(13) =1860318a(14) =14538245a(15) =122670593a(16) =1111715644a(17) =10771412394a(18) =111125142979