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Let t be a positive real number. A graph is called t-tough if the removal of any vertex set S that disconnects the graph leaves at most |S|/t components. The toughness of a graph is the largest t for which the graph is t-tough. A graph is minimally t-tough if the toughness of the graph is t, and the deletion of any edge from the graph decreases the toughness. Series–parallel graphs are graphs with two distinguished vertices called terminals, formed recursively by two simple composition operations, series and parallel joins. They can be used to model series and parallel electric circuits. We characterize minimally t-tough series–parallel graphs for all t > 1/2. It is clear that there is no minimally t-tough series– parallel graph if t > 1. We show that for 1 ≥ t > 1/2, most of the series–parallel graphs with toughness t are minimally t-tough. We conjecture that the situation is very different if the toughness is 1/2. We show a family of minimally 1/2-tough graphs, and conjecture that these are the only such graphs.
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