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Published online by Cambridge University Press: 24 October 2008
Let L ⊂ S3 be a knot or a link. A tunnel system for L is a collection of disjoint arcs t1, … ts properly embedded in S3−N(L) so that is a handlebody. Alternatively one can view the t1, … ts ascocores of 2-handles which have to be attached to a handlebody in order to obtain S3−N(L). It follows immediately that this handlebody is of genus s+1. The tunnel number of L, denoted by t(L), is defined to be the minimal cardinality of all such collections.