Let $M$ be a complete, simply connected Riemannian manifold with positive 
constant
scalar curvature, and $TM$ its tangent bundle with the lift metric I+II.
If $TM$ admits an essential infinitesimal conformal transformation, then 
$M$ is isometric to the
standard sphere. Furthermore if $M$ is compact, then the assumption 
``essential" is reduced to ``non-homothetic".