# Math Help - Standard notation

1. ## Standard notation

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".)

2. 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".)

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

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

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