  In this paper, we investigate the existence of positive solutions for a
nonlocal fractional boundary value problem involving Caputo fractional
derivative and nonlocal Riemann-Stieltjes integral boundary condition. By using
the spectral analysis of the relevant linear operator and Gelfand's formula, we
obtain an useful upper and lower bounds for the spectral radius. Our discussion
is based on the properties of the Green's function and the fixed point index
theory in cones.
