The notion of group is fundamental in our days, not only in mathematics,
but also in classical mechanics, electromagnetism, theory of relativity,
quantum mechanics, theory of elementary particles, etc. This notion has
developed during a century and this development is connected with the
names of great mathematicians as E. Galois, A. L. Cauchy, C. F. Gauss,
W. R. Hamilton, C. Jordan, S. Lie, E. Cartan, H. Weyl, E. Wigner, and of
many others. In mathematics, as in other sciences, the simple and
fertile ideas make their way with difficulty and slowly; however, this
long history would have been of a minor interest, had the notion of
group remained connected only with rather restricted domains of
mathematics, those in which it occurred at the beginning. But at
present, groups have invaded almost all mathematical disciplines,
mechanics, the largest part of physics, of chemistry, etc. We may say,
without exaggeration, that this is the most important idea that occurred
in mathematics since the invention of infinitesimal calculus; indeed,
the notion of group expresses, in a precise and operational form, the
vague and universal ideas of regularity and symmetry. The notion of
group led to a profound understanding of the character of the laws which
govern natural phenomena, permitting to formulate new laws, correcting
certain inadequate formulations and providing unitary and non-
contradictory formulations for the investigated phenomena.