  We show that the determinant $\nabla(d,\alpha),$ which seems to be not
considered in the past, is not zero. As an application of this result we prove
that the Setoyanagi operator $S_{p,q}=\partial^{2}-(ax^{p}+bx^{q}) $ is
irreducible over $\mathbb{C}(x) [ \partial] $.
