HP 50g Double factorial

[Gilles]

(05-01-2019 03:57 PM)joeres Wrote:  thanks for the answer with the short and sweet version.
I like that a question mark is displayed for negative numbers. However, at -1 and -2 not. This is certainly related to the formula?

Kind regards
Joerg

Yes it is. We can do :

Code:

«  -> n 
 «
  IF 'n<0 OR FP(n)' THEN  #203h DOERR ELSE
   'IFTE(n MOD 2,(n+1)!/(2^((n+1)/2)*((n+1)/2)!),2^(n/2)*(n/2)!)' ->NUM
  END 
 »
»

A negative or not integer input will result an error "Bad argument value".