Add neural networks

tikz/hopfield-network/Makefile

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+SOURCE = hopfield-network
+DELAY = 80
+DENSITY = 300
+WIDTH = 512
+
+make:
+	pdflatex $(SOURCE).tex -output-format=pdf
+	make clean
+
+clean:
+	rm -rf  $(TARGET) *.class *.html *.log *.aux *.data *.gnuplot
+
+gif:
+	pdfcrop $(SOURCE).pdf
+	convert -verbose -delay $(DELAY) -loop 0 -density $(DENSITY) $(SOURCE)-crop.pdf $(SOURCE).gif
+	make clean
+
+png:
+	make
+	make svg
+	inkscape $(SOURCE).svg -w $(WIDTH) --export-png=$(SOURCE).png
+
+transparentGif:
+	convert $(SOURCE).pdf -transparent white result.gif
+	make clean
+
+svg:
+	make
+	#inkscape $(SOURCE).pdf --export-plain-svg=$(SOURCE).svg
+	pdf2svg $(SOURCE).pdf $(SOURCE).svg
+	# Necessary, as pdf2svg does not always create valid svgs:
+	inkscape $(SOURCE).svg --export-plain-svg=$(SOURCE).svg
+	rsvg-convert -a -w $(WIDTH) -f svg $(SOURCE).svg -o $(SOURCE)2.svg
+	inkscape $(SOURCE)2.svg --export-plain-svg=$(SOURCE).svg
+	rm $(SOURCE)2.svg

tikz/hopfield-network/README.md

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+Compiled example
+----------------
+![Example](hopfield-network.png)

tikz/hopfield-network/hopfield-network.tex

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+\documentclass[varwidth=true, border=2pt]{standalone}
+\usepackage{tikz}
+
+\tikzstyle{neuron}=[draw,circle,minimum size=20pt,inner sep=0pt, fill=white]
+\tikzstyle{stateTransition}=[very thick]
+\tikzstyle{learned}=[text=red]
+
+\begin{document}
+    \newcommand\n{5}
+    \begin{tikzpicture}[scale=1.3]
+        \begin{scope}[rotate=17]
+        %the multiplication with floats is not possible. Thus I split the loop in two.
+        \foreach \number in {1,...,\n}{
+            \node[neuron] (N-\number) at ({\number*(360/\n)}:1.5cm) {$x_\number$};
+        }
+
+        \foreach \number in {1,...,\n}{
+            \foreach \y in {1,...,\n}{
+                \draw[stateTransition] (N-\number) -- (N-\y);
+            }
+        }
+        \end{scope}
+        \begin{scope}[rotate=-1]
+        \draw[learned,stateTransition] (N-1) -- (N-2) node [midway,above=-0.15cm,sloped] {$w_{1,2}$};
+        \draw[learned,stateTransition] (N-1) -- (N-5) node [midway,above=-0.15cm,sloped] {$w_{1,5}$};
+        \end{scope}
+    \end{tikzpicture}
+\end{document}