A maximal zero-free interval for chromatic polynomials of bipartite planar graphs

It is well known that image and image are two maximal zero-free intervals for all chromatic polynomials. Jackson [A zero-free interval for chromatic polynomials of graphs, Combin. Probab. Comput. 2 (1993), 325–336] discovered that image is another maximal zero-free interval for all chromatic polynomials. In this note, we show that image is actually a maximal zero-free interval for the chromatic polynomials of bipartite planar graphs.