\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[english,greek]{babel}
\usepackage{amsmath}
\usepackage{color}
\usepackage{tabularx}
\usepackage{tikz}
\usepackage[margin=1in]{geometry}
\renewcommand\tabularxcolumn[1]{m{#1}}
\newcolumntype{M}{>{\centering\arraybackslash}m{1cm}}
\newcommand\tikzmark[2]{%
\tikz[remember picture,baseline] \node[inner sep=2pt,outer sep=0] (#1){#2};%
}
\newcommand\link[2]{%
\begin{tikzpicture}[remember picture, overlay, >=stealth, shift={(0,0)}]
\draw[->] (#1) to (#2);