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

Terms

a(0) =0a(1) =2a(2) =8a(3) =34a(4) =136a(5) =546a(6) =2184a(7) =8738a(8) =34952a(9) =139810a(10) =559240a(11) =2236962a(12) =8947848a(13) =35791394a(14) =143165576a(15) =572662306a(16) =2290649224a(17) =9162596898a(18) =36650387592a(19) =146601550370a(20) =586406201480