Consider a second-order ordinary differential equation
 |
If 
and 
remain finite at 
, then 
is called an ordinary point.
If either 
or 
diverges as 
,
then 
is called a singular point. If either 
or 
diverges as 
but 
and 
remain finite as 
, then 
is called a regular singular point (or nonessential singularity).
