How do I use the Gamma Function in Excel
 

Hi!
I'm trying to use a formula that contains the Gamma function in Excel
but this does not seem to be under the list of functions. If I type its
definition:
Gamma(n)=(n-1)! it gives an error message when n-1 is not
an integer.
Has anyone got any ideas how I can get past this?
Thanks

There is no intrinsic GAMMA function in Excel, although there are GAMMADIST,
GAMMAINV, GAMMALN. I suggest that you look these up in help to see what they
do.

--

HTH

RP
(remove nothere from the email address if mailing direct)


"Cardiff Maths Student" <Cardiff Maths
wrote in message ...
Hi!
I'm trying to use a formula that contains the Gamma function in Excel
but this does not seem to be under the list of functions. If I type its
definition:
Gamma(n)=(n-1)! it gives an error message when n-1 is not
an integer.
Has anyone got any ideas how I can get past this?
Thanks

Based on Bob's suggestion, try this:

Put the number you want Gamma of in A1.

Put some other number in A2. "1" seems to work fine.

Put =GAMMADIST(A2,A1,1,FALSE) in A3

Then you can get Gamma(A1) with =EXP(-A2)*A2^(A1-1)/A3

This appears to work for positive A1.


Art

This may be easier than my previous post:

use =EXP(-1)/GAMMADIST(1,A1,1,FALSE) for Gamma(A1)

Again, this doesn't appear to work for negative A1.

Art

This is one way to get Gamma(A1):

=EXP(GAMMALN(A1))

It's been noted before that it's only good to about 9 or 10 digits of
accuracy.
--
Dana DeLouis
Win XP & Office 2003


"Bob Phillips" wrote in message
...
There is no intrinsic GAMMA function in Excel, although there are
GAMMADIST,
GAMMAINV, GAMMALN. I suggest that you look these up in help to see what
they
do.

--

HTH

RP
(remove nothere from the email address if mailing direct)


"Cardiff Maths Student" <Cardiff Maths
wrote in message
...
Hi!
I'm trying to use a formula that contains the Gamma function in Excel
but this does not seem to be under the list of functions. If I type its
definition:
Gamma(n)=(n-1)! it gives an error message when n-1 is
not
an integer.
Has anyone got any ideas how I can get past this?
Thanks

Thanks to all of you for your help! I think it's simplest to take the
exponential of the log of the Gamma function as Dana suggested, the degree of
accuarcy is sufficient for my project, thanks again!

Dana DeLouis wrote...
This is one way to get Gamma(A1):

=EXP(GAMMALN(A1))

It's been noted before that it's only good to about 9 or 10 digits
of
accuracy.

GAMMALN's poorer around 1 and 2, where it evaluates negative.

FWIW, the lngamma udf in the .xls file in following linked .zip file
does a reasonable job, accurate to 14+ decimal digits.

ftp://members.aol.com/hrlngrv/gammaln8.zip

Perhaps in a few more versions Microsoft might actually use the
publicly available (and not even GPLed) netlib code in all the
'engineering' and stats functions in Excel.
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