Some Boundedness and Unboundedness Results for the Equation x'' + a(t) x2 n-1 = 0. D. Willett — 1967 Monatshefte für Mathematik
On the oscillation of ty" + p(t)y = 0 with ʃ p almost periodic D. Willett — 1973 Annales Polonici Mathematici
On the oscillatory behavior of the solutions of second order linear differential equations D. Willett — 1969 Annales Polonici Mathematici
On the Discrete Analogues of Some Genralizations of Gronwall's Inequality. J.S.W. Wong; D. Willett — 1965 Monatshefte für Mathematik