a(n) = A299914(2n+1).

Terms

a(0) =1a(1) =6a(2) =42a(3) =306a(4) =2250a(5) =16578a(6) =122202a(7) =900882a(8) =6641514a(9) =48963042a(10) =360969210a(11) =2661166386a(12) =19618866954a(13) =144635805954