flag curvatures of the unit sphere in a minkowski-randers space

On a real vector space v , a randers norm fˆ is defined by fˆ = ˆα+βˆ, where ˆα is a euclidean norm and βˆ is a covector. we show that the unit sphere σ in the randers space (v, fˆ) has positive flag curvature, if and only if |βˆ|αˆ < (5 − √ 17)/2 ≈ 0.43845, thus answering a problem proposed by prof. zhongmin shen. moreover, we prove that the flag curvature of σ has a universal lower bound −4.