LINEAR DIFFERENTIAL EQUATIONS WITH EXCEPTIONAL FUNDAMENTAL SETS
We will prove the following conjecture of Wittich: Let L(w) = w(n) + pn−2(z)w + · · · + p0(z)w be a linear differential operator with polynomial coefficients and let {w1, . . . , wn} be a fundamental set of the equation L(w) = 0, each wj having Borel exceptional value 0 (or being a polynomial). Then the coefficients of L are constants.