  In this paper, we recall the basic results of the reduction theory of
positive definite quadratic forms. Since finding the shortest vectors in a
lattice is an NP-hard problem, the low-dimensional results in lattice reduction
theory have an important role. Using the result of Ryskov on admissible
centerings and the result of Tammela about the determination of a
Minkowski-reduced form, we prove that the absolute values of the coordinates of
a minimum vector in a six-dimensional Minkowski-reduced basis are less or equal
to three. To get this sharpening of P. Tammela's interesting works, we combine
some elementary geometric reasonings with the mentioned theoretical results.
