Add DL expressivity

src/deep-learning/sections/_expressivity.tex

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+\chapter{Neural networks expressivity}
+
+
+
+\section{Perceptron}
+
+Single neuron that defines a binary threshold through a hyperplane:
+\[
+    \begin{cases}
+        1 & \sum_{i} w_i x_i + b \geq 0 \\
+        0 & \text{otherwise}
+    \end{cases}    
+\]
+
+\begin{description}
+    \item[Expressivity] \marginnote{Perceptron expressivity}
+        A perceptron can represent a NAND gate but not a XOR gate.
+        \begin{center}
+            \begin{minipage}{.2\textwidth}
+                \centering
+                \includegraphics[width=\textwidth]{img/perceptron_nand.pdf}
+                \tiny NAND
+            \end{minipage}
+            \begin{minipage}{.2\textwidth}
+                \centering
+                \includegraphics[width=\textwidth]{img/xor.pdf}
+                \tiny XOR
+            \end{minipage}
+        \end{center}
+
+        \begin{remark}
+            Even if NAND is logically complete, the strict definition of a perceptron is not a composition of them.
+        \end{remark}
+\end{description}
+
+
+
+\section{Multi-layer perceptron}
+
+Composition of perceptrons.
+
+\begin{descriptionlist}
+    \item[Shallow neural network] \marginnote{Shallow NN}
+        Neural network with one hidden layer.
+
+    \item[Deep neural network] \marginnote{Deep NN}
+        Neural network with more than one hidden layer.
+\end{descriptionlist}
+
+\begin{description}
+    \item[Expressivity] \marginnote{Multi-layer perceptron expressivity}
+        Shallow neural networks allow to approximate any continuous function 
+        \[ f: \mathbb{R} \rightarrow [0, 1] \]
+
+        \begin{remark}
+            Still, deep neural networks allow to use less neural units.
+        \end{remark}
+\end{description}