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Re: product formula in mathematica
*To*: mathgroup at smc.vnet.net
*Subject*: [mg68767] Re: [mg68702] product formula in mathematica
*From*: Murray Eisenberg <murray at math.umass.edu>
*Date*: Fri, 18 Aug 2006 03:12:13 -0400 (EDT)
*Organization*: Mathematics & Statistics, Univ. of Mass./Amherst
*References*: <200608170818.EAA24861@smc.vnet.net>
*Reply-to*: murray at math.umass.edu
*Sender*: owner-wri-mathgroup at wolfram.com
In general, if you have a list lis of numbers, you can find their product by
Apply[Times, lis]
or, in abbreviated format:
Times @@ lis
So a strategy would be to form first the list of numbers to be
multiplies, and then use the above.
Now I'm not quite sure what you mean by x_i in your posting. Are you
trying to explicitly use subscripts (via the Notation package)? Indices
via the x[[i]] notation? Ordinary arguments via the x[i] notation.
In any case, since you have no particular pattern in mind for the
subscripts, you'll somehow have to specify them one-by-one. For example
(doing it slowly, step by step):
x = Table[i^2, {i, 1, 10}]
{1, 4, 9, 16, 25, 36, 49, 64, 81, 100}
indices = {1,3,4,9}
{1, 3, 4, 9}
x[[indices]]
{1, 9, 16, 81}
f[x_] := 1/x (* for example *)
f[x[[indices]]]
{1, 1/9, 1/16, 1/81}
Times @@ f[x[[indices]]]
1/11664
In the above, I'm using, and assuming you have, a function f that has
attribute Listable. (Also, I actually showed the InputForm of the two
results that have fractions in them.)
frou wrote:
> Hi I am interested in doing a product over a set of indices that are
> not continuous
> Product [ f(x_i) , { x_i, 1,3,4,9}] Is this possible it appears
> that the iterator wouldonly take sequences such as 1...10 etc.
>
> Thanks
>
>
--
Murray Eisenberg murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 413 549-1020 (H)
University of Massachusetts 413 545-2859 (W)
710 North Pleasant Street fax 413 545-1801
Amherst, MA 01003-9305
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