Introduction

During recent decades hundreds of thousands of new organic compounds have been synthesized every year, each of which may react with many others. This “jungle” of organic compounds continues to grow unremittingly. Therefore, new methods leading to better orientation in this “jungle” should be of great importance for introducing a logic and order into organic chemistry. Mathematics and mathematical models are now playing a principal role in these attempts. In general, the models may be classified as physical and nonphysical, where the difference between them is, however, of a very relative nature and the trend is towards a physicalization of nonphysical models. A typical representative of physical models is quantum chemistry. The nonphysical models are usually based on discrete mathematics, employing, in particular, graph theory, many different algebras, and group theory.