Bridge (graph theory)
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In graph theory, a bridge (also known as a cut-edge or an isthmus) is an edge whose deletion increases the number of connected components. Equivalently, an edge is a bridge if and only if it is not contained in any cycle.
A graph is said to be bridgeless if it contains no bridges. It easy to see that this is equivalent to 2-edge-connectivity.
An important open problem involving bridges is the so-called Cycle Double Cover Conjecture, due to Seymour and Szekeres (1978 and 1979, independently), which states that every bridgeless graph admits a set of cycles which contains each edge exactly twice.

