An open circle means the function is not defined at that point.
When graphing the interval 0 ≤ x < 1, you would put a filled-in circle at x=0 and an open circle at x=1, indicating x=1 is not part of the graph, but points very close to x=1 are part of the graph.
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Open circles are used for the same purpose on other kinds of graphs, too.
