Friedrichs extensions for Sturm-Liouville operators with complex coefficients and their spectra

In this paper, we study the Friedrichs extensions of Sturm-Liouville operators with complex coefficients according to the classification of B. M. Brown et al. [3]. We characterize the Friedrichs extensions both by boundary conditions at regular endpoint and asymptotic behaviours of elements in the maximal operator domains at singular endpoint. Some of spectral properties are also involved.