Jacobsthal oblong numbers.

Terms

a(0) =0a(1) =1a(2) =3a(3) =15a(4) =55a(5) =231a(6) =903a(7) =3655a(8) =14535a(9) =58311a(10) =232903a(11) =932295a(12) =3727815a(13) =14913991a(14) =59650503a(15) =238612935a(16) =954429895a(17) =3817763271a(18) =15270965703a(19) =61084037575a(20) =244335800775a(21) =977343902151