Leftmost column of array A275960.

Terms

a(0) =1a(1) =4a(2) =18a(3) =216a(4) =1794a(5) =16050a(6) =161748a(7) =1754886a(8) =20252058a(9) =257046648a(10) =9715067802a(11) =169069882650