_{i}} be an orthonormal basis of T

_{p}M and let b

_{ei}be the corresponding Busemann functions on M. Then we show that :

(1) The vector space V ={bv | v ∈ T_{p}M } is finite dimensional and
dim V = dim M = n.

(2) F : M → R^{n} defined by F(x) = (b_{e1}(x); b_{e2}(x); …; b_{en}(x)) is an isometry and
therefore, M is flat.

Thus, the flatness of M is shown by using the strongest
criterion.