A numerical Knot invariant. For a Tame Knot , the bridge index is the least Bridge Number of all planar representations of the Knot. The bridge index of the Unknot is defined as 1.
See also Bridge Number, Crookedness
References
Rolfsen, D. Knots and Links. Wilmington, DE: Publish or Perish Press, p. 114, 1976.
Schubert, H. ``Über eine numerische Knotteninvariante.'' Math. Z. 61, 245-288, 1954.