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  • Perplex
    IB Math AIHL
    /
    Graph Theory
    /

    Problem Bank

    [Maximum mark: 6]

    Thomas, a train enthusiast, wants to take every major train in America. He makes a graph with vertices representing cities and edges representing train rides. Trains only go one direction, so he uses a directed graph. Each edge has weight ​1.

    <p>The diagram shows six vertices, each representing a city, arranged roughly in a West–East layout:</p><p>• San Francisco (left)<br>• Los Angeles (below and slightly right of San Francisco)<br>• Chicago (to the right of Los Angeles and above center)<br>• New York (to the right of Chicago and slightly above center)<br>• Washington DC (to the right of Chicago and below New York)<br>• Boston (to the right of Chicago and above New York)</p><p>Directed edges (arrows) connect them as follows:</p><ol><li><p>From San Francisco there is one outgoing arrow pointing right to Chicago.</p></li><li><p>From Los Angeles, there are two outgoing arrows. One goes to San Francisco, and the other to Washington DC.</p></li><li><p>From Chicago, there are 3 outgoing arrows, which point to Boston, New York, and Los Angeles.</p></li><li><p>From New York, there is a single outgoing arrows to Los Angeles.</p></li><li><p>From Washington DC there is a single outgoing arrow, which points to New York.</p></li><li><p>From Boston there is a single outgoing arrow, which points to New York.</p></li></ol><p>No edge labels or weights are shown; only the city names and the directions of the arrows are indicated.</p>
    1. State whether there exists an Eulerian circuit for this graph. State your reasoning.

      [2]

    Thomas, the train enthusiast, lives in New York. Hence, he will start and end his train trip in New York.

    1. Find the shortest path from New York to Boston.

      [1]
    2. America Rail would like to make the graph contain an Eulerian circuit by flipping the directions of as few edges as possible.

      1. Find the edge that they should flip

        [2]
      2. Find an Eulerian circuit starting with the train ​NY→LA.

        [1]

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