A characterization of quaternionic projective space by the   conformal-Killing equation
  We prove that a compact quaternionic-K\"{a}hler manifold of dimension $4n\geq 8$ admitting a conformal-Killing 2-form which is not Killing, is isomorphic to the quaternionic projective space, with its standard quaternionic-K\"{a}hler structure. 
Liana David and Massimiliano Pontecorvo