I know what a factorial is, but I can't remember whether there is a
word and/or formula taking a number, lets say 5, and adding 5+4+3+2+1,
which is 15.  Does this function (that's probably not the right word,
I'm not a mathematition, have a word, like "factorial"?  If so, what
is it?

There is no special name for this function. But there is a simple formula:
1 + 2 + 3 + ... + n = n(n+1)/2 
The proof can be found here: http://mathworld.wolfram.com/ArithmeticSeries.html

These numbers are sometimes called triangular:
http://mathworld.wolfram.com/TriangularNumber.html

Thanks lophar-ga.  Quick aside question.  It seems like many of the
easier questions on GA are answered for free, in the comments.  What's
your incentive?  ARe you trying to become a Researcher, or just a nice
guy who stumbled upon my question?  Or is there another explanation. 
Thanks again.

The motivation varies from person to person.  Some people hope to
become one of the official paid researchers, some just like to help,
and some like to show how smart they are.

I answer questions just for fun when i have nothing else to do, and i
need no money for this.

OK.  Thanks to both of you.

Amitrangra-ga,
Your comment will probably be deleteed because you posted an email address.
That's no-no here.


Lawboy,
Here is a definition of factorial:

"In mathematics, the factorial of a natural number n is the product of
the positive integers less than or equal to n. This is written as n!
and pronounced "n factorial". The notation n! was introduced by
Christian Kramp in 1808."
en.wikipedia.org/wiki/Factorial

And here is another with an example:

"The product of a series of consecutive positive integers from 1 to a
given number (n). It is expressed with the symbol ( ! ). For example,
5! = 5x4x3x2x1 = 120. As a rule (n!+n) is evenly divisible by n."
dorakmt.tripod.com/mtd/glosmath.html