Studying power series and Frobenius method , I found a theorem stating that there is a unique Maclaurin series $y(x)$ satisfying the IVP $$y''+a(x)y'+b(x)y=0 ,\ \ y(0)=\alpha \ \ ,y'(0)=\beta$$ provided $a(x)$ and $b(x)$ are analytic at $x=0$ .
So does this mean that if we have $x=0$ is a regular singular point , we do not have always a unique solution for the IVP ? or not always , we may have a unique solution or not and we just do not guarantee the unique solution ?
For example , the problem $$xy''-xy'+y=e^{x}\ \ \ ,y(0)=1\ \ \ ,y'(0)=2$$ The general solution ( particular and homogeneous solutions) is $$y(x)=(e^x-x)+c_1x+c_2(-1+x\ln(x)+\frac{x^2}{2}+\frac{x^3}{12}+...)$$ (Note : the details of the solution is here Solving this non homogeneous IVP using power series) Applying initial conditions , we find that $c_1=2$ , $c_2=0$ (unique values for the constants although $x=0$ is singular point!)
However , another problem $$x^2y''-3xy'+3y=0,\ \ y(0)=0\ \ ,y'(0)=1$$ has $x=0$ a regular singular point , and has solution $$y=c_1x+c_2x^3$$ Applying intial conditions , we find that $c_1=1$ but $c_2$ has infinite values.