“…It has some similarities to the work of Levi [3], for the differential ideals [y p ] and [uv] as well as [2], [4], [5], and [7]. Although the Wronskian is zero if and only if {y { } is a linearly dependent set (the y'& being analytic functions) [8, p. 34], by the RittRaudenbush Theorem of Zeros [8, p. 27] , k d } is a fixed set of nonnegative integers (with possible repetitions), the set of polynomials in products of degree d, Vi 1 k ι -* y% d k d f°r any choice of the i, , is also a subspace of <% and & is the direct sum of these subspaces, (With d = 0, the subspace is F.) The intersection of these two gradings is the one we use, and we usually work in a subspace which is homogeneous with respect to both gradings.…”