1. A. Devinatz, Positive definite fourth-order differential operators, J. London Math. Soc. (2), 6(1973), 412–416.
2. W. D. Evans, On non-integrable square solutions of a fourth-order differential equation and the limit-2 classification, To appear in J. London Math. Soc.
3. W. N. Everitt, Some positive definite differential operators, J. London Math. Soc. 43 (1968), 465–473.
4. M. A. Naimark, Linear Differential Operators, GITTL, Moscow 1954; English transl. Ungar, New York, part I, 1967; part II, 1968.