HP 50g Double factorial

[DavidM]

(01-27-2024 05:57 AM)FLISZT Wrote:  ...(from ListExt library).

If you've got ListExt installed, there's a fairly simple way to do this if we are only interested in non-negative integers (ie. no gamma function required):

Code:

\<<
  { 2 - } { 1 > } LWHL
  LPROD
\>>

The first line simply builds the list of multiplicands, given n as the argument on stack level 1:
0 yields { }
1 yields { }
2 yields { 2 }
3 yields { 3 }
4 yields { 4 2 }
5 yields { 5 3 }
6 yields { 6 4 2 }
7 yields { 7 5 3 }
...

By definition, LPROD provides 1 as the result when given an empty list. That works nicely here. Also, using "1 >" as the check condition, the superfluous final "1" is skipped for multiplication with odd n.

Note that this is just the core of the function; if input validation is needed, that should be added as required.