# [seqfan] Re: more on double factorials/Penson

Maximilian Hasler maximilian.hasler at gmail.com
Sat Oct 24 23:21:26 CEST 2009

```On Sat, Oct 24, 2009 at 3:47 PM, N. J. A. Sloane <njas at research.att.com> wrote:
> I have been aware of the multiple definitions of double factorial
> for the whole time the database has existed, and I'm not worried
> if Maple disagrees with the OEIS.  I don't propose to make any changes.

It has been pointed out that Maple's doublefactorial() does NOT
disagree with the OEIS, but along with the general function
doublefactorial() = A006882: "Double factorials n!!",
OEIS has the two additional subsequences (2n)!! and (2n-1)!!
which are called "double factorial numbers" in their %N line.

> (More generally, I think Maple has had a very bad influence
> on mathematical notation - allowing horrors like A/B/C/D....
> If the OEIS and Maple disagree, so much the better!)

I (also) disagree ;-)
I like the effort made in Maple to allow for nice mathematical notation like
f := x -> sin(x)
and I find it horrible to see people starting to write
(outside the context of a specific programming language)
Sin[x] when they mean sin(x),
{ ... } when they mean a list and not a set, etc.

The notation A/B/C is not at all specific to Maple,
this is used since the earliest days of FORTRAN programming ;
and everyday people use A - B - C to know what is left if they had A
and have spent B and C,
which is exactly the same than to do A / B / C for knowing what is
left if you divide by B and then by C.

(What I do dislike in the context of OEIS + Maple are things like
A001654:=1/(z+1)/(z**2-3*z+1);
which is very bad for many reasons - but this is not the fault of
Maple in itself.... ;-)

Maximilian
PS: when the OEIS goes wiki, someone will certainly, sooner or later,
add "odd" in the %N A001147 (=(2n-1)!!), and no-one will undo that
change ;-)

```