Standard notation

**courteous** · November 27th 2009, 10:09 AM

Are these below a "standard" (i.e. not-obscure) piece one can see in textbooks:

$n_{(r)}=\frac{n^{(r)}}{r!}$ [ $n_{(3)}=\frac{n(n-1)(n-2)}{3!}$ ] and $n_{r}=\frac{n^r}{r!}$

( $n!!$ , I presume, is "standard".)

Pretty please post more "alike".

**apcalculus** · November 27th 2009, 02:22 PM

Originally Posted by courteous

Are these below a "standard" (i.e. not-obscure) piece one can see in textbooks:

$n_{(r)}=\frac{n^{(r)}}{r!}$ [ $n_{(3)}=\frac{n(n-1)(n-2)}{3!}$ ] and $n_{r}=\frac{n^r}{r!}$

( $n!!$ , I presume, is "standard".)

Pretty please post more "alike".

The factorial notation is "standard", sure. Sorry, I may be missing the point here. Are you asking if this notation is typical?

**emakarov** · November 27th 2009, 04:03 PM

I am not sure I've seen the subscript and superscript notation.

**Plato** · November 27th 2009, 04:52 PM

Originally Posted by emakarov

I am not sure I've seen the subscript and superscript notation.

I definitely agree with that.

It appears to me as if the is a combination of the standard usage of permutations and factorials.
By that I mean $_n\mathcal{P}_j=\frac{n!}{(n-j)!}$ so that $_{10}\mathcal{P}_3=(10)(9)(7)$ .

So that $n_{(r)}=\frac{_n\mathcal{P}_r }{r!}$ .

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