How to expand a summation with different indexes

1 次查看（过去 30 天）

显示更早的评论

SooShiant 2015-10-2

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/246428-how-to-expand-a-summation-with-different-indexes

评论： Thorsten 2015-10-6

How to expand a summation with different indexes like this?

Any help would be appreciated.

4 个评论
显示 2更早的评论隐藏 2更早的评论

Stephen23 2015-10-2

@SooShiant: Please stop posting the same question again and again.

SooShiant 2015-10-2

@Stephen Cobeldick

That was different. This is numerical but that one you deleted was symbolical. Don't you know the symbolical answer?

请先登录，再进行评论。

请先登录，再回答此问题。

回答（4 个）

Thorsten 2015-10-2

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/246428-how-to-expand-a-summation-with-different-indexes#answer_194476

在 MATLAB Online 中打开

sum(diff(f(1:4)).*n(1:3).*arrayfun(@(x) sum(m(x:end)), 3:5))

@Thorsten : nice solution exploiting fact that each sum is independent of the other index, i and j, we may use dot products to compute the inner loop, and repeat it again for index i. I wonder if there is also a way to do it with mesgrid() and matrix products.

SooShiant 2015-10-2

@Muthu Annamalai: don't you know the symbolical code?

请先登录，再进行评论。

Sean de Wolski 2015-10-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/246428-how-to-expand-a-summation-with-different-indexes#answer_194473

编辑：Sean de Wolski 2015-10-2

在 MATLAB Online 中打开

sum(diff(f(1:4)).*n(1:3).*sum(m(3:5)))

5 个评论
显示 3更早的评论隐藏 3更早的评论

Thorsten 2015-10-2

编辑：Thorsten 2015-10-2

Sean, yes, i is 1:3, so the starting index of j is 3:5, so the sum runs for j = 3:5 for i = 1, j = 4:5 for i = 2, and j = 5 for i = 3, i.e., assumes values sum(m(3:5)), sum(m(4:5)) and sum(m(5)) for i=1,2,3. You forgot to sum over the different m(j), as far as I can see.

Sean de Wolski 2015-10-2

Ah, so I need to tril() that part.

请先登录，再进行评论。

SooShiant 2015-10-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/246428-how-to-expand-a-summation-with-different-indexes#answer_194479

As I said I want a code to expand summation (algebraic) not to get a numerical answer. How bad is it. There is no way. May I need to try Mathematica. Can any one help with Mathematica?

3 个评论
显示 1更早的评论隐藏 1更早的评论

SooShiant 2015-10-2

I've seen this page before but as I said I'm new to matlab and need an emergency help. May you write down this code for me pleeeeeease??????????

Thorsten 2015-10-6

Unfortunately I don't have the symbolic toolbox.

请先登录，再进行评论。

SooShiant 2015-10-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/246428-how-to-expand-a-summation-with-different-indexes#answer_194496

I need Matlab answer me:

I wonder how you experts can't writes such a simple code. May it relates to Matlab capabilities and should use Mathematica. Can anyone help with Mathematica?