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I have been reading a research paper. The author has used a variable '$x$' with an underline beneath '$x$'. I know that a variable with a line on top of it implies it's arithmetic mean. But I have never seen a symbol with a line under the variable name. Can anyone please tell what it means?

Thanks in advance.

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    $\begingroup$ You mean $\large{\underline x}$ ? What is the context ? $\endgroup$ May 16, 2016 at 18:16
  • $\begingroup$ Yes. But the paper is related to Linear Programming. $\endgroup$
    – sajid
    May 16, 2016 at 18:17
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    $\begingroup$ There is no universal meaning for underlining of variables. It would be impossible to guess the meaning without having the paper identified, but almost surely the author defines that notation before using it. $\endgroup$
    – hardmath
    May 16, 2016 at 18:18
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    $\begingroup$ "have been reading a research paper" - where? A link would be helpful. Even better, as previously noted, the author surely explained his/her notations somewhere near the paper's beginning. $\endgroup$ May 16, 2016 at 18:20
  • $\begingroup$ @user3708999 Can you scan the context and upload? $\endgroup$
    – Narasimham
    May 16, 2016 at 18:27

4 Answers 4

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It's not an often-used convention, but in physics, matrices are sometimes appended with a double line underneath and vectors a single line underneath. This somewhat unifies the matrix/vector notation without the clumsiness of vector notation (and how to extend that to matrices).

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    $\begingroup$ Your answer gives me a hint that it should be a vector. It fits in the context. Thanks. $\endgroup$
    – sajid
    May 16, 2016 at 18:27
  • $\begingroup$ The fact that it's easy to use this notation in handwriting makes it very popular, I think. $\endgroup$
    – Edward
    Nov 11, 2021 at 10:20
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In the article linked below, it seems it is used to mean the minimum possible value of the variable, while the same variable with a line above it means the maximum possible value of the variable.

https://www.researchgate.net/publication/221107546_On_Decision_Making_under_Interval_Uncertainty_A_New_Justification_of_Hurwicz_Optimism-Pessimism_Approach_and_its_Use_in_Group_Decision_Making

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Such a notation often means a vector that is a solution of minimization problem in linear programming.

I googled for another article on MDS problem, it uses just ordinary vector notation (see p.4)

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My teacher (Statistics) said it is a symbol representing the series of observations$$x_1,x_2,x_3,x_4,\ldots$$in order to reduce space and time.

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