What does the mathematical symbol with an underline below the variable name mean? 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.
 A: 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).
A: 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
A: 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) 
A: 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.
