Problem of the $c\to\infty$ limit in the relativistic Schrödinger equation

A mathematical procedure is given for investigating the regular degeneration of the solutions of the relativistic Schrödinger equation
$$ [2c\sqrt{q^2+m^2c^2}-H_0^{\operatorname{rad}}-V(r)]\Psi_{ql}(r)=0 $$
into the solutions of the nonrelativlstle equation
$$ \left[\hbar^2\frac{d^2}{dr^2}-\hbar^2\frac{l(l+1)}{r^2}-mV(r)+q^2\right]u_{ql}(r)=0 $$
for the $S$-wave case. The proposed method of a small parameter of the higher derivatives of a differential equation is applied to several concrete problems.