A Pu–Bonnesen inequality

We prove an inequality of Bonnesen type for the real projective plane, generalizing Pu’s systolic inequality for positively-curved metrics. The remainder term in the inequality, analogous to that in Bonnesen’s inequality, is a function of R - r (suitably normalized), where R and r are respectively the circumradius and the inradius of the Weyl–Lewy Euclidean embedding of the orientable double cover. We exploit John ellipsoids of a convex body and Pogorelov’s ridigity theorem.