1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sigma notation and parentheses question

  1. Jun 28, 2009 #1
    I have a question about sigma notation and parentheses. Does

    [tex]\sum x[k] + x[n][/tex]


    [tex]\sum( x[k] + x[n])[/tex] or [tex]\left(\sum x[k] \right)+ x[n][/tex]

    I find it a personal annoyance for things to be written with parentheses even if there are rules about it. Including parentheses would just reduce ambiguity. Another time I sometimes get confused is when trig functions are written without parentheses - grrrrrrr.
    Last edited: Jun 28, 2009
  2. jcsd
  3. Jun 28, 2009 #2


    User Avatar
    Homework Helper

    Without more context I can see two possibilities.

    a) It is [tex] \left( \sum_k x[k] \right) + x[n] [/tex]

    b) It is [tex] \sum_{k, n} (x[k] + x[n]) [/tex]

    I think 'a' is the more reasonable, but I base that solely on a hunch. Is there more to the original article?
  4. Jun 28, 2009 #3
    I worked out the problem it was in and it was in fact 'a'. I'm wondering if there is some rule or if you just have to look and sorta figure it out based on the context?
    Last edited: Jun 28, 2009
  5. Jun 28, 2009 #4


    User Avatar
    Science Advisor

    It is probably (a) but parentheses surely should have been used!
  6. Jun 28, 2009 #5


    User Avatar
    Homework Helper

    My clue for going with 'a' as my most likely choice was the single summation symbol but [tex] x [/tex] occurring twice with different indices.

    It was an incredibly poorly written expression - in what context was it (math, physics,?)
  7. Jun 29, 2009 #6
    Don't know if you still care, but it was in my electrical engineering signals and systems book. The book was pretty much talking about how it is possible to rewrite a sum as the sum up to a certain number by the sum up to that last (number - 1) with the final value of the signal added. This was in discreet time obviously. It seems really obvious when I state it here, but in the book they jumped a few steps which made it even harder to figure out what that equation was saying.
  8. Jun 29, 2009 #7


    User Avatar
    Homework Helper

    Thanks. You will find poorly formatted items in texts and articles from almost every discipline, but that is little consolation when you have to make an attempt to decode the author's (or authors', if multiple) carelessness.
  9. Jun 29, 2009 #8
    This actually is a little consolation because I heard that electrical engineers are especially bad about not using rigorous mathematics and using lots of shortcuts. I hope notation is not one of those.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook