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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$\begingroup$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).
$\endgroup$ 2 $\begingroup$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.
$\endgroup$ $\begingroup$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)
$\endgroup$ $\begingroup$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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