In a lens space X of order r a knot K representing an element of the fundamental group 1 X Š =ޚr ޚ of order s Ä r contains a connected orientable surface S properly embedded in its exterior X N .K/ such that @S intersects the meridian of K minimally s times. Assume S has just one boundary component. Let g be the minimal genus of such surfaces for K , and assume s 4g 1. Then with respect to the genus one Heegaard splitting of X , K has bridge number at most 1.
57M27; 57M25
Statement of resultsAny knot K in a lens space X D L.r; q/, r > 0, is rationally nullhomologous, ie OEK D 0 2 H 1 .X I /ޑ Š 0. We say r is the order of the lens space X , and we say the smallest positive integer s such that sOEK D 0 2 H 1 .X I /ޚ Š =ޚr ޚ is the order of the knot K . Note s Ä r . The exterior X N .K/ of K thus contains a connected properly embedded orientable surface S such that when S is oriented @S is coherently oriented on @ x N .K/ and intersects the meridian  @ x N .K/ of K minimally s times, ie j @S j D s . Such a surface S is an analogue of a Seifert surface for a knot in S 3 . We refer to the genus of a knot K in X as the minimal genus of these "rational" Seifert surfaces for K . For this article we will restrict our attention to knots with rational Seifert surfaces that have just one boundary component.In this paper we prove the following theorem. Theorem 1.1 Let K be a genus g knot of order s in a lens space X whose Seifert surfaces have one boundary component. If s 4g 1 then, with respect to the Heegaard torus of X , K has bridge number at most 1. Theorem 1.1 may be curiously rephrased as saying small genus knots in lens spaces have small bridge number.In [1] Berge shows that double-primitive knots (ie simple closed curves that lie on a genus 2 Heegaard surface in S 3 and represent a generator of 1 for each handlebody)