Column k=1 of A213221.

Terms

a(0) =1a(1) =3a(2) =10a(3) =32a(4) =106a(5) =357a(6) =1222a(7) =4230a(8) =14770a(9) =51918a(10) =183472a(11) =651191a(12) =2319626a(13) =8288135a(14) =29691760a(15) =106611596a(16) =383565754a(17) =1382423748a(18) =4990274016a(19) =18039252192a(20) =65292492328a(21) =236595647526a(22) =858231115540