Even terms in A002181.

Terms

a(0) =33817088a(1) =67634176a(2) =135268352a(3) =270536704a(4) =541073408a(5) =1082146816a(6) =2164293632a(7) =4328587264a(8) =8589934592a(9) =11734529536a(10) =17179869184a(11) =23469059072a(12) =34359738368a(13) =46938118144a(14) =68719476736a(15) =83764926976a(16) =93876236288a(17) =137438953472a(18) =161882400256a(19) =162288205312a(20) =167529853952a(21) =229246039552a(22) =230869259776a(23) =231883772416a(24) =239391165952a(25) =245072436736