  In this work, we investigate the spectral problem $Au = {\lambda}u$ where $A$
is a fractional elliptic operator involving left- and right-sided
Riemann-Liouville derivatives. These operators are nonlocal and nonsymmetric,
however, share certain classic elliptic properties. The eigenvalues correspond
to the roots of a class of certain special functions. Compared with classic
Sturm-Liouville problems, the most challenging part is to set up the framework
for analyzing these nonlocal operators, which requires developing new tools. We
prove the existence of the real eigenvalues, find the range for all possible
complex eigenvalues, explore the graphs of eigenfunctions, and show numerical
findings on the distribution of eigenvalues on the complex plane.
