[1]  Dunford N, Schwartz J T. Linear Operators (II). New York: Wiley-interscience, 1963

[2]  Sims A R. Secondary conditions for linear differential operators of the second order. J Math Mech, 1957, 6: 247--285

[3]  Brown B M, McCormack  D K R,  Evans W D, Plum M. On the spectrum of second-order differential operators with complex coefficients. Proc R Soc Lond Ser, 1999, A(455): 1235--1257

[4]  Lesch M,  Malamud M. On the deficiency indices and self-adjointness of symmetric Hamiltonian systems. J Differential Equations, 2003,  189: 556--615

[5]  Hille E.  Lectures on Ordinary Differential Equations. London: Addison-Weyl, 1969

[6]  Read T T. A limit-point criterion for expression with oscillatory coefficients. Pacific J Math, 1976, 66: 143--255

[7]  Atkinson F V.  Discrete and Continuous Boundary Problems. New York: Academic Press, 1964

[8] Qi Jiangang. Limit-point criterion for singular linear Dirac differential systems. Computers and Mathematics with Applications, 2005, 49: 765--775