Partial sums of A020988.

Terms

a(0) =0a(1) =2a(2) =12a(3) =54a(4) =224a(5) =906a(6) =3636a(7) =14558a(8) =58248a(9) =233010a(10) =932060a(11) =3728262a(12) =14913072a(13) =59652314a(14) =238609284a(15) =954437166a(16) =3817748696a(17) =15270994818a(18) =61083979308a(19) =244335917270a(20) =977343669120