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Let K be a knot that has an unknotting tunnel τ. We prove that K admits a strong involution that fixes τ pointwise if and only if K is a two-bridge knot and τ its upper or lower tunnel.
We show that twisted torus knots T(p, q; 3, s) are tunnel number one. A short spanning arc connecting two adjacent twisted strands is an unknotting tunnel.
We show that the degenerations of tunnel numbers of knots under connected sum can be arbitrarily large.