# Workshop Selection

Let \(D\) be the set of all possible days to hold a workshop and let \(P\) be the set of people interested in attending the workshop such that each person \(p_{i}\) wants to attend during \([s_{i}, f_{i}] \subset D\). Each day \(d_{i}\) costs \(c_{i}\), but each person \(p_{i}\) will willing pay \(v_{i}\) to attend. **Suppose there is no limit to the number of people that can attend on a particular day, what days should you select to maximize profit?**

Constrct a flow network \(N = (V, E)\) as follows:

- Create a source vertex \(s \in V\) and a sink vertex \(t \in V\)
- Create a vertex for each person \(p_{i} \in P\)
- Create a vertex for each day \(d_{j} \in D\)
- Create a directed edge \((s, p_{i})\) of capacity \(v_{i}\) for all \(p_{i} \in P\)
- Create a directed edge \((d_{i}, t)\) of capacity \(c_{i}\) for all \(d_{i} \in D\)
- Create a directed edge \((p_{i}, d_{j})\) of capacity \(\infty\) for all \(p_{i}\) such that \(d_{j} \in [s_{i}, f_{i}]\)

For some subset \(U \subset V\), let \(rev(U) = \sum\nolimits_{p_{i} \in U} v_{p}\) and \(cost(U) = \sum\nolimits_{d_{i} \in U} c_{i}\). Clearly, \(profit(U) = rev(U) - cost(U)\).

Find the minimum cut \(S, T\) of \(N\). Because all \(s-t\) paths contain edges of finite capacity by construction, the minimum cut must always be finite. Therefore, the cut-set of the minimum cut \(S, T\) may only contain edges of the form \((s, p_{i})\) or \((d_{i}, t)\). Because no edges may cross the cut, all the desired days for each person in \(S\) must be in \(S\). Consequently, the cut capacity is defined as:

\[c(S, T) = \sum\nolimits_{p_{i} \in T} v_{i} + \sum\nolimits_{d_{j} \in S} c_{j} = rev(T) + cost(S)\]Therefore,

\[\begin{align*}profit(S) &= rev(S) - cost(S) \\ &= rev(V) - (rev(T) + cost(S)) \\ &= rev(V) - c(S, T)\end{align*}\]Because \(c(S, T)\) is minimal, \(profit(S)\) must be maximal. Therefore, the days \(d_{i} \in S\) generate the maximal profit of \(rev(V) - c(S, T)\).

# Detecting Arbitrage

Let \(C\) be a set of currencies and let the \(R\) be the set of exchange rates between currencies such that \(r(c_{i}, c_{j}) \in R\) is the exchange rate between currencies \(c_{i}\) and \(c_{j}\). Clearly, \(r(c_{i}, c_{j}) = \frac{1}{r(c_{j}, c_{i})}\). Arbitrage occurs when there are opportunities for riskless profit. **How do you detect arbitrage opportunities between currencies?**

In the graph \(G = (C, R)\) arbitrage occurs where there exists a cycle of currencies \(c_{1},c_{2},\ldots,c_{k},c_{1}\) such that:

\[\begin{align*}r(c_{1}, c_{2}) r(c_{2}, c_{3}) \ldots r(c_{k-1}, c_{k}) r(c_{k}, c_{1}) &> 1\\ \log r(c_{1}, c_{2}) + \log r(c_{2}, c_{3}) + \ldots + \log r(c_{k-1}, c_{k}) + \log r(c_{k}, c_{1}) &> 0\\ \log \frac{1}{r(c_{1}, c_{2})} + \log \frac{1}{r(c_{2}, c_{3})} + \ldots + \log \frac{1}{r(c_{k-1}, c_{k})} + \log \frac{1}{r(c_{k}, c_{1})} &< 0\end{align*}\]Therefore, detecting arbitrage opportunities in \(G\) is equivalent to finding negative cost cycles in the graph \(G' = (C, \{ \log \frac{1}{r} : r \in R\})\), which can be found in \(O(\lvert C \rvert ^{3})\) using the Floyd-Warshall Algorithm.

# Tiling

**How many ways can one tile a 3 x n rectangle using 2 x 1 tiles?** It is immediately clear that \(n\) must be even, because it is impossible to tile an odd area with even area tiles. Therefore, \(n = 2 \cdot k\) and \(k \in \mathbb{N}\). By enumerating the various ways to tile 2 x 1 tiles on a 3 x \(2 \cdot k\) rectangle for \(k \in \{1, 2, 3\}\), I determined that the number of tilings:

By subtraction of these two equations,

\[\begin{align*} T(k) - T(k - 1) &= [3 \cdot T(k - 1) + 2 \cdot T(k - 2) + ... + 2 \cdot T(0)] - \\ &\ \qquad [3 \cdot T(k - 2) + 2 \cdot T(k - 3) + ... + 2 \cdot T(0)] \\ &= 3 \cdot T(k - 1) - T(k - 2)\end{align*}\]Therefore, \(T(k) = 4 \cdot T(k - 1) - T(k - 2)\). The characteristic equation of this linear homogeneous recurrence relation is \(r^{2} = 4 \cdot r - 1\). The roots of this characteristic equation are \(a_{1} = \frac{4 + \sqrt{12}}{2} = 2 + \sqrt{3}\) and \(a_{2} = \frac{4 - \sqrt{12}}{2} = 2 - \sqrt{3}\). Suppose the closed-form solution of the recurrence relation is of the form

\[T(k) = c_{1} a_{1}^{k} + c_{2} a_{2}^{k} = c_{1} (2 + \sqrt{3})^{k} + c_{2} (2 - \sqrt{3})^{k}\] \[\begin{align*}T(0) = 1 \implies 1 &= c_{1} + c_{2} \implies c_{2} = 1 - c_{1} \\ T(1) = 3 \implies 3 &= c_{1} (2 + \sqrt{3}) + c_{2} (2 - \sqrt{3}) \\ &= c_{1} (2 + \sqrt{3}) + (1 - c_{1}) (2 - \sqrt{3}) \\ &= 2 \sqrt{3} c_{1} + 2 - \sqrt{3}\end{align*}\]Consequently, \(c_{1} = \frac{1 + \sqrt{3}}{2 \sqrt{3}}\) and \(c_{2} = 1 - \frac{1 + \sqrt{3}}{2 \sqrt{3}} = \frac{\sqrt{3} - 1}{2 \sqrt{3}}\). Therefore, the closed-form solution to the recurrence relation is \(T(k) = \frac{1 + \sqrt{3}}{2 \sqrt{3}} (2 + \sqrt{3})^{k} + \frac{\sqrt{3} - 1}{2 \sqrt{3}} (2 - \sqrt{3})^{k}\) and solves the problem in \(O(1)\) time and \(O(1)\) space.

**How many ways can one tile a k x n rectangle using 2 x 1 tiles?** Assuming we can tile the first n-1 columns of the rectangle, how many ways are there to tile the last column? We can either tile elements in the last column one-at-a-time using a horizontal tile or two-at-a-time using a vertical tile. Therefore, the number of ways to tile the last column \(f(k) = f(k-2) + f(k-1)\). Because the \(n^{th}\) Fibonnaci number \(F(n) = \frac{\phi^{n}}{\sqrt{5}}\), the number of ways to tile the last column grows exponentially with \(k\). Because there are \(n\) columns in the matrix, the complexity of the algorithm is \(O(\phi^{nk})\).

Cover photograph by aspireblog.