A026899 T(2n+1,n+2), T given by A026780.
1, 6, 30, 144, 687, 3296, 15972, 78294, 388320, 1947910, 9875566, 50560246, 261177401, 1360112590, 7134802722, 37673830960, 200106632766, 1068539351340, 5733226876876, 30894846668416, 167137498306038, 907416216735816
Offset: 1