How can I write this letters (in blue) in LaTeX ? its look like $z$ 
Some help would be appreciated
Here the code for your formula.
\documentclass{article}
\usepackage{newtxtext,newtxmath}
\begin{document}
\[
\mathbf{z} = \{ z_{ij}; i = 1, 2, \ldots, n_a; j = 1, 2, \ldots, n_a - i + 1 \}
\]
\end{document}
times is obsolete. Also, to keep things minimal, I removed the 12pt font class option and the use of mathtools.
– Werner
Aug 16 '18 at 19:40
\mathbf{z}and italic z with subscript italic ij isz_{ij}, but none of these are Greek letters? – Au101 Aug 16 '18 at 19:11