\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[english,spanish]{babel}
\usepackage[width=16cm,height=20cm]{geometry}
\usepackage{amsmath,amssymb}
\newcommand{\bT}{\mathbb{T}}
\newcommand{\Circulant}{\operatorname{Circ}}
\newcommand{\diag}{\operatorname{diag}}

\usepackage{tikz}
\usetikzlibrary{trees,arrows,fadings,mindmap,shapes,decorations.pathreplacing}

\definecolor{greenlinecolor}{rgb}{0.3,0.7,0.3}
\definecolor{themecolor}{rgb}{0.9,0.9,1}
\definecolor{strongcolor}{rgb}{0.8,1,0.8}
\definecolor{auxiliarcolor}{gray}{0.3}

\begin{document}


\noindent
El siguiente esquema muestra los conceptos
y resultados principales de este capítulo.
\begin{center}
\begin{tikzpicture}
[outer sep=3pt,
ar/.style={
  line width=2pt,greenlinecolor
},
auxnode/.style={
  rectangle,rounded corners,text centered,
  minimum size=1cm,text width=4.5cm,
  fill=themecolor,draw=greenlinecolor
},
mainnode/.style={
  rectangle,rounded corners,text centered,
  minimum size=1.5cm,text width=5cm,
  fill=strongcolor,draw=greenlinecolor
}]
\node[auxnode] (Fourier) at (-3,0)
  {Transf. finita de Fourier,\\
  $F_n$ y $\Phi_n=\frac{1}{\sqrt{n}}F_n$};
\node[auxnode] (Convolution) at (3,0)
  {Convolución\\discreta cíclica, $\ast$};
\node[auxnode] (ThmConv) at (-3,-3)
  {Teorema de convolución:\\$F_n(a\ast b)=(F_n a)\odot(F_n b)$};
\node[auxnode] (CircMatr) at (3,-3)
  {Matriz circulante,\\$\Circulant(a)$};
\node[mainnode] (ThmEigCircMatr) at (0,-6)
  {Diagonalización de\\matrices circulantes:\\[0.5ex]
  $\Phi^\ast \Circulant(a) \Phi_n = \diag(F_n a)$
  }; 
\draw[ar,-stealth] (Fourier)--(ThmConv);
\draw[ar,-stealth] (Convolution)--(ThmConv);
\draw[ar,-stealth] (Convolution)--(CircMatr);
\draw[ar,-stealth] (ThmConv)--(ThmEigCircMatr);
\draw[ar,-stealth] (CircMatr)--(ThmEigCircMatr);
\end{tikzpicture}
\end{center}

\end{document}