books.google.com - This book is about metric spaces of nonpositive curvature in the sense of Busemann, that is, metric spaces whose distance function satisfies a convexity condition. The book also contains a systematic introduction to the theory of geodesics in metric spaces, as well as a detailed presentation of some...http://books.google.com/books/about/Metric_Spaces_Convexity_and_Nonpositive.html?id=JrwzXZB0YrIC&q=implies&utm_source=gb-gplus-shareMetric Spaces, Convexity and Nonpositive Curvature