Numbers of the form (4^k - 4)/k.

Terms

a(0) =0a(1) =6a(2) =20a(3) =63a(4) =204a(5) =682a(6) =2340a(7) =381300a(8) =1398101a(9) =5162220a(10) =71582788a(11) =1010580540a(12) =14467258260