diff --git a/Advanced/Graph Algorithms/main.tex b/Advanced/Graph Algorithms/main.tex new file mode 100755 index 0000000..a76d686 --- /dev/null +++ b/Advanced/Graph Algorithms/main.tex @@ -0,0 +1,180 @@ +% https://git.betalupi.com/Mark/latex-packages +% use [nosolutions] flag to hide solutions. +% use [solutions] flag to show solutions. +% Last built with version 1.1.0 +\documentclass[ + solutions +]{ormc_handout} + +\usepackage{subfiles} +\usetikzlibrary{arrows.meta} +\usetikzlibrary{shapes.geometric} + +% We put nodes in a separate layer, so we can +% slightly overlap with paths for a perfect fit +\pgfdeclarelayer{nodes} +\pgfdeclarelayer{path} +\pgfsetlayers{main,nodes} + +% Layer settings +\tikzset{ + % Layer hack, lets us write + % later = * in scopes. + layer/.style = { + execute at begin scope={\pgfonlayer{#1}}, + execute at end scope={\endpgfonlayer} + }, + % + % Arrowhead tweaks + >={Latex[ width=2mm, length=2mm ]}, + label/.style = { + circle, + % For automatic red background in solutions + fill = \ORMCbgcolor, + draw = none + }, + % + % Nodes + main/.style = { + draw, + circle, + fill = white + }, + % + % Flow annotations + flow/.style = { + opacity = 1, + thin, + inner xsep = 2.5mm, + inner ysep = 2.5mm + }, + % + % Paths + path/.style = { + line width = 4mm, + draw = black, + % Lengthen paths so they're + % completely under nodes. + line cap = rect, + opacity = 0.3 + } +} + +\begin{document} + + \maketitle + + + {Algorithms on Graphs: Flow} + + { + Prepared by Mark on \today + } + + + \subfile{parts/00 review} + \subfile{parts/01 flow} + \subfile{parts/02 residual} + \subfile{parts/03 fulkerson} + + + + + + \problem{Maximum Cardinality Matching} + + A \textit{matching} is a subset of edges in a bipartite graph. Nodes in a matching must not have more than one edge connected to them. \\ + A matching is \textit{maximal} if it has more edges than any other matching. + + \vspace{5mm} + + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + Initial Graph \\ + \vspace{2mm} + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (A1) at (0mm, 24mm) {}; + \node[main] (A2) at (0mm, 18mm) {}; + \node[main] (A3) at (0mm, 12mm) {}; + \node[main] (A4) at (0mm, 6mm) {}; + \node[main] (A5) at (0mm, 0mm) {}; + \node[main] (B1) at (20mm, 24mm) {}; + \node[main] (B2) at (20mm, 18mm) {}; + \node[main] (B3) at (20mm, 12mm) {}; + \node[main] (B4) at (20mm, 6mm) {}; + \node[main] (B5) at (20mm, 0mm) {}; + \end{scope} + + % Edges + \draw + (A1) edge (B2) + (A1) edge (B3) + (A2) edge (B1) + (A2) edge (B4) + (A4) edge (B3) + (A2) edge (B3) + (A5) edge (B3) + (A5) edge (B4) + ; + + \end{tikzpicture} + \end{center} + \end{minipage} + \hfill + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + Maximal Matching \\ + \vspace{2mm} + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (A1) at (0mm, 24mm) {}; + \node[main] (A2) at (0mm, 18mm) {}; + \node[main] (A3) at (0mm, 12mm) {}; + \node[main] (A4) at (0mm, 6mm) {}; + \node[main] (A5) at (0mm, 0mm) {}; + \node[main] (B1) at (20mm, 24mm) {}; + \node[main] (B2) at (20mm, 18mm) {}; + \node[main] (B3) at (20mm, 12mm) {}; + \node[main] (B4) at (20mm, 6mm) {}; + \node[main] (B5) at (20mm, 0mm) {}; + \end{scope} + + % Edges + \draw[opacity = 0.4] + (A1) edge (B2) + (A1) edge (B3) + (A2) edge (B1) + (A2) edge (B4) + (A4) edge (B3) + (A4) edge (B3) + (A5) edge (B3) + (A5) edge (B4) + ; + \draw + (A1) edge (B2) + (A2) edge (B1) + (A4) edge (B3) + (A5) edge (B4) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + + \vspace{5mm} + + Devise an algorithm to find a maximal matching in any bipartite graph. \\ + Find an upper bound for its runtime. + + \begin{solution} + Turn this into a maximum flow problem and use FF. \\ + Connect a node $S$ to all nodes in the left group and a node $T$ to all nodes in the right group. All edges have capacity 1. + + \vspace{2ex} + + Just like FF, this algorithm will take at most $\min(\# \text{ left nodes}, \# \text{ right nodes})$ iterations. + \end{solution} + +\end{document} \ No newline at end of file diff --git a/Advanced/Graph Algorithms/parts/00 review.tex b/Advanced/Graph Algorithms/parts/00 review.tex new file mode 100755 index 0000000..9a489b2 --- /dev/null +++ b/Advanced/Graph Algorithms/parts/00 review.tex @@ -0,0 +1,91 @@ +\documentclass[../main.tex]{subfiles} + + +\begin{document} + + \section{Review} + + \definition{} + A \textit{graph} consists of a set of \textit{nodes} $\{A, B, ...\}$ and a set of edges $\{ (A,B), (A,C), ...\}$ connecting them. + A \textit{directed graph} is a graph where edges have direction. In such a graph, $(A, B)$ and $(B, A)$ are distinct edges. + A \textit{weighted graph} is a graph that features weights on its edges. \\ + A weighted directed graph is shown below. + + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope} + \node[main] (A) {$A$}; + \node[main] (B) [below right of = A] {$B$}; + \node[main] (C) [below left of = A] {$C$}; + \end{scope} + + % Edges + \draw[->] + (A) edge[bend right] node[label] {$4$} (B) + (B) edge node[label] {$2$} (C) + (C) edge node[label] {$2$} (A) + (B) edge[bend right] node[label] {$1$} (A) + ; + \end{tikzpicture} + \end{center} + + \vfill + + \definition{} + We say a graph is \textit{bipartite} if its nodes can be split into two groups $L$ and $R$ so that no two nodes in the same group are connected by an edge. \\ + The following graph is bipartite: + + \begin{center} + \begin{tikzpicture} + % Nodes + \begin{scope} + \node[main] (A) at (0mm, 0mm) {$A$}; + \node[main] (B) at (0mm, -10mm) {$B$}; + \node[main] (C) at (0mm, -20mm) {$C$}; + + \node[main] (D) at (20mm, 0mm) {$D$}; + \node[main] (E) at (20mm, -10mm) {$E$}; + \node[main] (F) at (20mm, -20mm) {$F$}; + \end{scope} + + % Edges + \draw + (A) edge (D) + (A) edge (E) + (B) edge (F) + (C) edge (E) + (C) edge (D) + ; + \end{tikzpicture} + \end{center} + + \vfill + + \definition{} + We say two nodes $A$ ane $B$ are \textit{connected} if we can reach $A$ from $B$ and $B$ from $A$ by walking along (possibly directed) edges. We say a graph is connected if all its nodes are connected to each other.\\ + + The bipartite graph above and the directed graph below are not connected. + + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope} + \node[main] (A) {$A$}; + \node[main] (B) [below right of = A] {$B$}; + \node[main] (C) [below left of = A] {$C$}; + \end{scope} + + % Edges + \draw[->] + (A) edge[bend right] (B) + (B) edge[bend right] (A) + (B) edge (C) + ; + \end{tikzpicture} + \end{center} + + \vfill + \pagebreak + +\end{document} \ No newline at end of file diff --git a/Advanced/Graph Algorithms/parts/01 flow.tex b/Advanced/Graph Algorithms/parts/01 flow.tex new file mode 100755 index 0000000..f15cc61 --- /dev/null +++ b/Advanced/Graph Algorithms/parts/01 flow.tex @@ -0,0 +1,363 @@ +\documentclass[../main.tex]{subfiles} + + +\begin{document} + + \section{Network Flow} + + \generic{Networks} + Say have a network: a sequence of pipes, a set of cities and highways, an electrical circuit, server infrastructure, etc. + + \vspace{1ex} + + We'll represent our network with a connected directed weighted graph. If we take a city, edges will be highways and cities will be nodes. There are a few conditions for a valid network graph: + + \begin{itemize} + \item The weight of each edge represents its capacity, the number of lanes in the highway. + \item Edge capacities are always positive integers.\hspace{-0.5ex}\footnotemark{} + \item Node $S$ is a \textit{source}: it produces flow. + \item Node $T$ is a \textit{sink}: it consumes flow. + \item All other nodes \textit{conserve} flow. In other words, the sum of flow coming in must equal the sum of flow going out. + \end{itemize} + + \footnotetext{An edge with capacity zero is equivalent to an edge that does not exist; An edge with negative capacity is equivalent to an edge in the opposite direction.} + + Here is an example of a such a graph: + \begin{center} + \begin{tikzpicture} + + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$4$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + \end{tikzpicture} + \end{center} + + \hrule{} + + \generic{Flow} + In our city example, cars represent \textit{flow}. Let's send one unit of cars along the topmost highway: + + \vspace{2ex} + + \begin{minipage}{0.33\textwidth} + \begin{center} + \begin{tikzpicture} + + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$4$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + + % Flow + \draw[path] + (S) -- node[above left, flow] {$(1)$} (A) + -- node[above right, flow] {$(1)$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + \begin{minipage}{0.65\textwidth} + There are a few things to notice here: + \begin{itemize} + \item Highlighted edges carry flow. + \item Numbers in parentheses tell us how much flow each edge carries. + \item The flow along an edge is always positive or zero. + \item Flow comes from $S$ and goes towards $T$. + \item Flow is conserved: all flow produced by $S$ enters $T$. + \end{itemize} + \end{minipage} + + \vspace{1ex} + + The \textit{magnitude} of a flow\footnotemark{} is the number of \say{flow-units} that go from $S$ to $T$. \\ + + We are interested in the \textit{maximum flow} through this network: what is the greatest amount of flow we can push from $S$ to $T$? + + \footnotetext{you could also think of \say{flow} as a directed weighted graph on top of our network.} + + \problem{} + What is the magnitude of the flow above? + + \vfill + \pagebreak + + \problem{} + Find a flow with magnitude 2 on the graph below. + + \begin{center} + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$4$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + \end{tikzpicture} + \end{center} + + \vfill + + \problem{} + Find a maximal flow on the graph below. \\ + \hint{The total capacity coming out of $S$ is 3, so any flow must have magnitude $\leq 3$.} + + \begin{center} + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$4$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + \end{tikzpicture} + \end{center} + + \vfill + \pagebreak + + \section{Combining Flows} + It is fairly easy to combine two flows on a graph. All we need to do is add the flows along each edge. For example, consider the following flows: + + \vspace{2ex} + + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + \begin{tikzpicture} + + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$2$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + + % Flow + \draw[path] + (S) -- node[above left, flow] {$(1)$} (A) + -- node[above right, flow] {$(1)$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + \hfill + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + \begin{tikzpicture} + + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$2$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + + % Flow + \draw[path] + (S) + -- node[below left, flow] {$(1)$} (B) + -- node[left, flow] {$(1)$} (A) + -- node[above right, flow] {$(1)$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + + \vspace{1cm} + + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + Combining these, we get the following: + \vspace{2ex} + + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (+00mm, +00mm) {$S$}; + \node[main] (A) at (+15mm, +15mm) {$A$}; + \node[main] (B) at (+15mm, -15mm) {$B$}; + \node[main] (T) at (+30mm, +00mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$2$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + + % Flow + \draw[path] + (S) + -- node[below left, flow] {$(1)$} (B) + -- node[left, flow] {$(1)$} (A) + -- node[above right, flow] {$(2) = (1) + (1)$} (T) + (S) + -- node[above left, flow] {$(1)$} (A) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + \hfill + \begin{minipage}[t]{0.48\textwidth} + \raggedright + When adding flows, we must respect edge capacities. + + \vspace{1ex} + + For example, we could not add these graphs if the magnitude of flow in the right graph above was 2. + + \vspace{1ex} + + This is because the capacity of the top-right edge is 2, and $2 + 1 > 2$. + \end{minipage} + + \vspace{2ex} + \hrule + \vspace{2ex} + + \problem{} + Combine the following flows and ensure that the flow along all edges remains within capacity. + + \vspace{2ex} + + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (C) [right of = A] {$C$}; + \node[main] (D) [right of = B] {$D$}; + \node[main] (T) [above right of = D] {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$5$} (A) + (A) edge node[label] {$3$} (C) + (C) edge node[label] {$2$} (T) + (A) edge node[label] {$4$} (D) + (S) edge node[label] {$4$} (B) + (B) edge node[label] {$1$} (D) + (D) edge node[label] {$2$} (T) + ; + + % Flow + \draw[path] + (S) + -- node[above left, flow] {$(2)$} (A) + -- node[above, flow] {$(2)$} (C) + -- node[above right, flow] {$(2)$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + \hfill + \begin{minipage}[t]{0.48\textwidth} + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (C) [right of = A] {$C$}; + \node[main] (D) [right of = B] {$D$}; + \node[main] (T) [above right of = D] {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$5$} (A) + (A) edge node[label] {$3$} (C) + (C) edge node[label] {$2$} (T) + (A) edge node[label] {$4$} (D) + (S) edge node[label] {$4$} (B) + (B) edge node[label] {$1$} (D) + (D) edge node[label] {$2$} (T) + ; + + % Flow + \draw[path] + (S) + -- node[above left, flow] {$(2)$} (A) + -- node[above right, flow] {$(2)$} (D) + -- node[below right, flow] {$(2)$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + + \vfill + \pagebreak + +\end{document} \ No newline at end of file diff --git a/Advanced/Graph Algorithms/parts/02 residual.tex b/Advanced/Graph Algorithms/parts/02 residual.tex new file mode 100755 index 0000000..a937f0c --- /dev/null +++ b/Advanced/Graph Algorithms/parts/02 residual.tex @@ -0,0 +1,219 @@ +\documentclass[../main.tex]{subfiles} + + +\begin{document} + + \section{Residual Graphs} + As our network gets bigger, finding a maximum flow by hand becomes much more difficult. It will be convenient to have an algorithm that finds a maximal flow in any network. + + \vspace{1ex} + + The first thing we'll need to construct such an algorithm is a \textit{residual graph}. + + \vspace{2ex} + \hrule + + \begin{center} + \begin{minipage}[t]{0.48\textwidth} + We'll start with the following network and flow: + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (T) [above right of = B] {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$1$} (A) + (A) edge node[label] {$3$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + + % Flow + \draw[path] + (S) -- node[above left, flow] {$(1)$} (A) + -- node[above right, flow] {$(1)$} (T) + ; + + \end{tikzpicture} + \end{center} + \end{minipage} + \hfill + \begin{minipage}[t]{0.48\textwidth} + First, we'll copy all nodes and \say{unused} edges: + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (T) [above right of = B] {$T$}; + \end{scope} + + % Edges + \draw[->] + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + \end{center} + + \hrule + + \begin{center} + \begin{minipage}[t]{0.48\textwidth} + Then, we'll add the unused capacity of \say{used} edges: (Note that $3 - 1 = 2$) + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (T) [above right of = B] {$T$}; + \end{scope} + + % Edges + \draw[->] + (A) edge node[label] {$2$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + \end{tikzpicture} + \end{center} + \end{minipage} + \hfill + \begin{minipage}[t]{0.48\textwidth} + Finally, we'll add \say{used} capacity as edges in the opposite direction: + \begin{center} + \begin{tikzpicture}[node distance = 20mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (T) [above right of = B] {$T$}; + \end{scope} + + % Edges + \draw[->] + (A) edge node[label] {$1$} (S) + (T) edge [bend right] node[label] {$1$} (A) + (A) edge [bend right] node[label] {$2$} (T) + (B) edge node[label] {$2$} (A) + (S) edge node[label] {$2$} (B) + (B) edge node[label] {$1$} (T) + ; + \end{tikzpicture} + \end{center} + This graph is the residual of the original flow. + \end{minipage} + \end{center} + + \hrule + \vspace{3ex} + + You can think of the residual graph as a \say{list of possible changes} to the original flow. \\ + There are two ways we can change a flow: + \begin{itemize} + \item We can add flow along a path + \item We can remove flow along another path + \end{itemize} + + \vspace{1ex} + + A residual graph captures both of these actions, showing us where we can add flow (forward edges) and where we can remove it (reverse edges). Note that \say{removing} flow along an edge is equivalent to adding flow in the opposite direction. + + + \vfill + \pagebreak + + \problem{} + Construct the residual of this flow. + + \begin{center} + \begin{tikzpicture}[node distance = 25mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (T) [above right of = B] {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$2$} (A) + (A) edge node[label] {$1$} (T) + (A) edge node[label] {$3$} (B) + (S) edge node[label] {$1$} (B) + (B) edge node[label] {$2$} (T) + ; + + % Flow + \draw[path] + (S) + -- node[above left, flow] {$(2)$} (A) + -- node[left, flow] {$(2)$} (B) + -- node[below right, flow] {$(2)$} (T) + ; + \end{tikzpicture} + \end{center} + + \begin{solution} + \begin{center} + \begin{tikzpicture}[node distance = 25mm] + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) {$S$}; + \node[main] (A) [above right of = S] {$A$}; + \node[main] (B) [below right of = S] {$B$}; + \node[main] (T) [above right of = B] {$T$}; + \end{scope} + + % Edges + \draw[->] + (A) edge node[label] {$2$} (S) + (A) edge node[label] {$1$} (T) + (A) edge[out=295,in=65] node[label] {$1$} (B) + (B) edge[out=115,in=245] node[label] {$2$} (A) + (S) edge node[label] {$1$} (B) + (T) edge node[label] {$2$} (B) + ; + \end{tikzpicture} + \end{center} + \end{solution} + + \vfill + \problem{} + Is the flow in \ref{FindResidual} maximal? \\ + If it isn't, find a maximal flow. \\ + \hint{Look at the residual graph. Can we add flow along another path?} + + \vfill + \pagebreak + + \problem{} + Show that... + \begin{enumerate} + \item A maximal flow exists in every network with integral\footnotemark{} edge weights. + \item Every edge in this flow carries an integral amount of flow + \end{enumerate} + + \footnotetext{Integral = \say{integer} as an adjective.} + + \vfill + + +\end{document} \ No newline at end of file diff --git a/Advanced/Graph Algorithms/parts/03 fulkerson.tex b/Advanced/Graph Algorithms/parts/03 fulkerson.tex new file mode 100755 index 0000000..260448a --- /dev/null +++ b/Advanced/Graph Algorithms/parts/03 fulkerson.tex @@ -0,0 +1,128 @@ +\documentclass[../main.tex]{subfiles} + + +\begin{document} + + \section{The Ford-Fulkerson Algorithm} + We now have all the tools we need to construct an algorithm that finds a maximal flow. \\ + It works as follows: + \begin{enumerate} + \item[\texttt{00}] Take a weighted directed graph $G$. + \item[\texttt{01}] Find any flow $F$ in $G$ + \item[\texttt{02}] Calculate $R$, the residual of $F$. + \item[\texttt{03}] ~~~~If $S$ and $T$ are not connected in $R$, $F$ is a maximal flow. \texttt{HALT}. + \item[\texttt{04}] Otherwise, find another flow $F_0$ in $R$. + \item[\texttt{05}] Add $F_0$ to $F$ + \item[\texttt{06}] \texttt{GOTO 02} + \end{enumerate} + + \problem{} + Run the Ford-Fulkerson algorithm on the following graph. \\ + There is extra space on the next page. + + \begin{center} + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (-5mm, 0mm) {$S$}; + \node[main] (A) at (20mm, 20mm) {$A$}; + \node[main] (B) at (20mm, 0mm) {$B$}; + \node[main] (C) at (20mm, -20mm) {$C$}; + \node[main] (D) at (50mm, 20mm) {$D$}; + \node[main] (E) at (50mm, 0mm) {$E$}; + \node[main] (F) at (50mm, -20mm) {$F$}; + \node[main] (T) at (75mm, 0mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$8$} (A) + (S) edge node[label] {$7$} (B) + (S) edge node[label] {$4$} (C) + + (A) edge node[label] {$2$} (B) + (B) edge node[label] {$5$} (C) + + (A) edge node[label] {$3$} (D) + (A) edge node[label] {$9$} (E) + (B) edge node[label] {$6$} (E) + (C) edge node[label] {$7$} (E) + (C) edge node[label] {$2$} (F) + + (E) edge node[label] {$3$} (D) + (E) edge node[label] {$4$} (F) + + (D) edge node[label] {$9$} (T) + (E) edge node[label] {$5$} (T) + (F) edge node[label] {$8$} (T) + ; + + \end{tikzpicture} + \end{center} + + \begin{solution} + The maximum flow is $17$. + \end{solution} + + \vspace{5mm} + + \pagebreak + + \begin{center} + \begin{tikzpicture} + % Nodes + \begin{scope}[layer = nodes] + \node[main] (S) at (-5mm, 0mm) {$S$}; + \node[main] (A) at (20mm, 20mm) {$A$}; + \node[main] (B) at (20mm, 0mm) {$B$}; + \node[main] (C) at (20mm, -20mm) {$C$}; + \node[main] (D) at (50mm, 20mm) {$D$}; + \node[main] (E) at (50mm, 0mm) {$E$}; + \node[main] (F) at (50mm, -20mm) {$F$}; + \node[main] (T) at (75mm, 0mm) {$T$}; + \end{scope} + + % Edges + \draw[->] + (S) edge node[label] {$8$} (A) + (S) edge node[label] {$7$} (B) + (S) edge node[label] {$4$} (C) + + (A) edge node[label] {$2$} (B) + (B) edge node[label] {$5$} (C) + + (A) edge node[label] {$3$} (D) + (A) edge node[label] {$9$} (E) + (B) edge node[label] {$6$} (E) + (C) edge node[label] {$7$} (E) + (C) edge node[label] {$2$} (F) + + (E) edge node[label] {$3$} (D) + (E) edge node[label] {$4$} (F) + + (D) edge node[label] {$9$} (T) + (E) edge node[label] {$5$} (T) + (F) edge node[label] {$8$} (T) + ; + + \end{tikzpicture} + \end{center} + + \vfill + \pagebreak + + \problem{} + You are given a large network. How would you quickly find an upper bound for the number of iterations the Ford-Fulkerson algorithm will need to find a maximum flow? + + \begin{solution} + Each iteration adds at least one unit of flow. So, we will find a maximum flow in at most $\min(\text{flow out of } S,~\text{flow into } T)$ iterations. + + \vspace{2ex} + + A simpler answer could only count the flow on $S$. + + \end{solution} + + \vfill + \pagebreak +\end{document} \ No newline at end of file