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Summation in matlab

  1. Oct 30, 2009 #1
    1. The problem statement, all variables and given/known data

    I want to calculate a sum (where the end value is in the sum), eg:
    [tex]\sum^{n-1}_{i=1}{2i+n}[/tex]

    2. Relevant equations

    I dont want to 'split' the sum, i just want to write this.

    3. The attempt at a solution

    syms i n
    for n=1:5
    for i=1:n-1
    symsum((2*i+n),i,1,n-1)
    end
    end

    i get the error: ??? Undefined function or method 'symsum' for input arguments of type 'double'.

    i think my code is wrong.
     
  2. jcsd
  3. Oct 30, 2009 #2

    Mark44

    Staff: Mentor

    Here's something to try:

    Code (Text):

    n = sym('n', r);
    for n = 1:5
       for i = 1:n-1
          symsum((2*sym('i', r) + n), i, n-1)
       end
    end
     
    No guarantee this will work, since I don't have matlab to test it out on. What I did is patterned after the last example on the reference page in the following link. I have also rewritten your syms command to use the sym command. The r flag specifies that both i and n are rational. Without a flag, they default to complex.
    Here's a link to a reference page for symsum.
    Here's a link to a page for sym.
    From the TOC that appears along with the reference page for sym, the next reference page is for syms.

    I don't think you have run into it, yet, since you're still trying to get the symsum/syms/sym thing sorted out, but I believe your inside loop will cause problems for you. When n = 1, the inner loop looks like for i=1, 0. The documentation for for doesn't go into as much detail as I would like, so isn't clear what happens when the starting value is larger than the ending value, and you haven't specified a negative increment. Possibly the loop skips that iteration. Don't know.

    Anyway, some things to think about and try out.
     
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