Third diagonal of triangle in A046739.

Terms

a(0) =1a(1) =21a(2) =161a(3) =813a(4) =3361a(5) =12421a(6) =42865a(7) =141549a(8) =453905a(9) =1426725a(10) =4422913a(11) =13579309a(12) =41408833a(13) =125667333a(14) =380081105a(15) =1146795693a(16) =3454279345a(17) =10392196645a(18) =31238454241a(19) =93845384301a(20) =281808780641a(21) =845996765061