HP 50g Double factorial

[FLISZT]

Hello

Just for the sake of using SEQ and ΠLIST:

Code:

<< → n
    << n 2 MOD
       { :: n :: n                 @ if n is even: 
         1 n 2 SEQ }                @ a list from 1 to n is created. 
       { :: n 1 − :: n             @ if n is odd:
         0 n 2 SEQ                  @ a list from '-1' to 'n − 1' is created.
         1 ADD                       @ adds 1 to each element of the list
         DUP                           @ duplicates the list (now from 0 to n)
         NOT                             @ new list: 0 becomes 1 ; others values become 0 
         ADD } IFTE                        @ adds the 2 lists, element by element
       ΠLIST                       @ returns the product of the elements of the list = the double factorial of n
    >>
>>

To avoid the {1} bug with ΠLIST, ΠLIST can be replaced by LPROD (from ListExt library).

:: are used to replace ' ' which can not be used in a list.

Or more conventionally, ' ' within chevrons:

<< 'n' … >> << 'n − 1' … >> IFTE