on generalized berwald manifolds with relatively isotropic landsberg curvature

The class of generalized berwald metrics contains the class of berwald metrics as a special case. let f= αφ(s), s=β/α, be a generalized berwald (α, β)-metric on manifold m. we show that f has vanishing s-curvature  s=0 and is of relatively isotropic landsberg curvature  l+ cfc=0 if and only if b=0, where c=c(x) is a scalar function on m.