a(0)=1, a(n)=4a(n-1)+2 for n odd, a(n)=4a(n-1) for n even.

Terms

a(0) =1a(1) =6a(2) =24a(3) =98a(4) =392a(5) =1570a(6) =6280a(7) =25122a(8) =100488a(9) =401954a(10) =1607816a(11) =6431266a(12) =25725064a(13) =102900258a(14) =411601032a(15) =1646404130a(16) =6585616520a(17) =26342466082a(18) =105369864328a(19) =421479457314