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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 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 ? $\endgroup$ – callculus May 16 '16 at 18:16
  • $\begingroup$ Yes. But the paper is related to Linear Programming. $\endgroup$ – sajid May 16 '16 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 '16 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. $\endgroup$ – J. M. is a poor mathematician May 16 '16 at 18:20
  • $\begingroup$ @user3708999 Can you scan the context and upload? $\endgroup$ – Narasimham May 16 '16 at 18:27
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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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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 '16 at 18:27
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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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