Even terms in A015922.

Terms

a(0) =2a(1) =4a(2) =8a(3) =248a(4) =17608a(5) =90148a(6) =106978a(7) =253828a(8) =364808a(9) =1642798a(10) =1926088a(11) =6176264a(12) =21879938a(13) =30484408a(14) =34634428a(15) =96593698a(16) =134396408a(17) =223622468a(18) =283585928a(19) =327327388a(20) =457961188a(21) =809965148a(22) =1709420344a(23) =1815124028a(24) =2164392968a(25) =2360006456a(26) =2431619908a(27) =2500777828a(28) =2922255548a(29) =3888155428