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What does the term "pointwise" refer to?

  1. Sep 7, 2015 #1
    In reading mathematical texts (especially on functions), I come across the term "pointwise". There seems to be a paucity of information explaining it on the internet, so I turn to the forum for an intuitive explanation. The context is when the term is used as in "pointwise addition" or "pointwise multiplication" of functions.
     
  2. jcsd
  3. Sep 7, 2015 #2

    SteamKing

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    This often refers to a graphical method of adding or multiplying the ordinates of two curves to obtain a third curve, which is the sum or product of the first two curves.

    Examine:

    pic7_10.gif
    You take a series of points from curves ch.1 and ch.2 at the same x-location and add them together algebraically to make the bottom curve in the graphic above.
     
  4. Sep 7, 2015 #3

    mathman

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    Let f,g,h be functions. f+g=h means f(x)+g(x)=h(x) for every x in the domain of these function. Similarly for any other operation involving functions. The term is more often used in the context of limits, where [itex]f_n -> f[/itex] can be defined in different ways, one of which is pointwise.
     
  5. Sep 7, 2015 #4
    So pointwise basically means that the way we manipulate functions is by considering their values at any x?
     
  6. Sep 9, 2015 #5
    Bump...
     
  7. Sep 9, 2015 #6

    Mark44

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    No, at a specific x value.

    "Pointwise" is commonly used in discussions about the convergence of sequences. See https://en.wikipedia.org/wiki/Pointwise_convergence for more info. One example given in this wiki article is the sequence ##\{x^n\}## on the interval [0, 1). This sequence converges pointwise to 0 for each x in [0, 1), but does not converge uniformly to 0 on the same interval.
     
    Last edited: Sep 9, 2015
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