The authors investigate the singular linear differential equation
on , where the functions and can have singularities at and . This means that and are not integrable on . Equation (1) is studied with the boundary conditions
where are certain interior points in . The authors introduce the Fredholm property for these problems which means that the unique solvability of the corresponding homogeneous problem implies the unique solvability of the nonhomogeneous problem for every which is weith-integrable on . Then, for the solvability of a problem having the Fredholm property, it sufficies to show that the corresponding homogeneous problem has only the trivial solution. In this way, the authors prove main theorems on the existence of a unique solution of (1),(2) and of (1),(3). Examples verifying the optimality of the conditions in various corollaries are shown as well.