# Regular solution vs irregular solution

My Quantum Mechanics textbook (John S. Townsend's A Modern Approach to Quantum Mechanics) mentions regular solutions and irregular solutions. It claims that regular solutions (at the origin) to the spherical Bessel equation are called spherical Bessel functions, while its irregular solutions (at the origin) are called spherical Neumann functions.

What's the difference between "regular" and "irregular"? And, if possible, please also tell me what "regular" and "irregular" solutions are in general (i.e.: please don't just tell me how they differ without illuminating what they are in the first place).

See below for a photo of the relevant page in my textbook. The stuff I'm asking about is found below eqn. 10.67.

I think that a regular solution $$y(k,r)$$ is one that satisfies the boundary conditions, $$y(k,0) = 0$$ $$y'(k,0) = 1,$$ while a irregular solution does not. However, there may be more to the answer than this.