title: Aleksandrov–Rassias problem
text: # Aleksandrov–Rassias problem
  
  The theory of isometries in the framework of Banach spaces has its beginning in a paper by Stanisław Mazur and Stanisław M. Ulam in 1932. They proved the Mazur–Ulam theorem stating that every isometry of a normed real linear space onto a normed real linear space is a linear mapping up to translation. In 1970, Aleksandr Danilovich Aleksandrov asked whether the existence of a single conservative distance for a mapping implies that it is an isometry. Themistocles M. Rassias posed the following problem:
  
     Aleksandrov–Rassias Problem. If X and Y are normed linear spaces and if T : X → Y is a continuous and/or surjective mapping such that whenever vectors x and y in X satisfy $\lVert x-y\rVert =1$, then $\lVert T(X)-T(Y)\rVert =1$ (the distance one preserving property or DOPP), is T then necessarily an isometry?
  
  There have been several attempts in the mathematical literature by a number of researchers for the solution to this problem.
  
  