  In this note we show that one open four dimensional ellipsoid embeds
symplectically into another if and only the ECH capacities of the first are no
larger than those of the second. This proves a conjecture due to Hofer. The
argument uses the equivalence of the ellipsoidal embedding problem with a ball
embedding problem that was recently established by McDuff. Its method is
inspired by Hutchings' recent results on embedded contact homology (ECH)
capacities but does not use them.
