A.D. Alexandrov's problem for Busemann non-positively curved spaces
  The paper is the last in the cycle devoted to the solution of Alexandrov's problem for non-positively curved spaces. Here we study non-positively curved spaces in the sense of Busemann. We prove that if $X$ is geodesically complete connected at infinity proper Busemann space, then it has the following characterization of isometries. For any bijection $f:X\to X$, if $f$ and $f^{-1}$ preserve the distance 1, then $f$ is an isometry. 
P.D. Andreev