[Date Index]
[Thread Index]
[Author Index]
Re: Some questions about vector
*To*: mathgroup at smc.vnet.net
*Subject*: [mg74554] Re: Some questions about vector
*From*: Bill Rowe <readnewsciv at sbcglobal.net>
*Date*: Mon, 26 Mar 2007 02:03:10 -0500 (EST)
On 3/25/07 at 1:26 AM, qzhang81 at gmail.com (Qi Zhang) wrote:
>input:
>Clear[f];
>f[v] = Sum[(i*v[i]), {i, 1, N}]
I am suprised the code above does anything meaningful
The summand in the code is the product of an index variable i,
and a function v of that variable that is no where defined. And
you have defined i to range from 1 to N which apparently Sum
recognizes as a variable despite the fact Mathematica has a
system object N which converts numeric quantities to machine precision.
You should notice that Sum[(i*v[i]), {i, 1, N}] is returned unevaluated.
And finally you define another function f with argument v (which
you are using to be a function on the rhs) to be the computed sum
>D[f[v], v[i]]
This code will evaluate f[v] to be the evaluated sum above.
Since each term of that sum is i*v[i] the derivative of each
term is i. Hence the result is simply the sum of integers from 1
to N which is what you obtained.
>the output is
>\!\(1\/2\ N\ \((1 + N)\)\)
>However what I expect is i
Why? What were you trying to do?
>What should I do in order to get the the answer I expected?
Most likely the answer is to use appropriate Mathematica syntax
--
To reply via email subtract one hundred and four
Prev by Date:
**Re: notebook as TXT**
Next by Date:
**Re: Suggestion for direct commands ParametricPlot22 and ParametricPlot33**
Previous by thread:
**Re: Some questions about vector**
Next by thread:
**Re: Some questions about vector**
| |