From Wigner--In{\"o}n{\"u} Group Contraction to Contractions of   Algebraic Structures
  The development of the notion of group contraction first introduced by E. In{\"o}n{\"u} and E.P. Wigner in 1953 is briefly reviewed. The fundamental role of the idea of degenerate transformations is stressed. The significance of contractions of algebraic structures for exactly solvable problems of mathematical physics is noticed. 
N.A. Gromov