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Very brief summnation question

  1. Dec 2, 2014 #1
    If there is a sum:
    i=1, and ends at N

    Why, if there is an N, is it considered as a constant.
    As opposed to an i inside of a summnation.

    I would understand that if for example, there were a Z, Q,E, or W, it's a non-variable.
    But N, that's ON top of the sum!
    Wouldn't it.. like turn into soemthing? Can anyone explain thoroughly?
  2. jcsd
  3. Dec 3, 2014 #2

    Stephen Tashi

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    Science Advisor

    Whether a symbol is a constant depends on the full context surrounding it's use. In the majority of typical textbook problems and discussions the [itex] N [/itex] in an expression like [itex] \sum_{i=1}^N a_i [/itex] represents a constant. But there is no absolute rule that [itex] N [/itex] must be used that way. For example, a person could say "Consider the function [itex] F(N) = \sum_{i=1}^N (2i^2 + 6) [/itex]", using [itex] N [/itex] as a variable.
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