Associate Legendre Function
Press[1]
gives a short stable code for calculating these. He also gives a warning that "most of
the recurrences involving m are unstable, and so dangerous for numerical
work. The stable recurrence is on 
This is useful because
The double factorial is defined by n!! is the product of all odd integers less than or equal to n.
Using with 
and setting 
, we find
Thus and provide a starting point for .
The actual code is very short.