This work is of interest to those who find in mathematics the universal language with which they can explain the phenomena that occur in our environment and, of course, for those who see in it a door that will lead them to the search for knowledge. oriented to scientific and technological development. Ángel Ruiz's prologue measures the virtues of this text in the context of teacher training and its positive impact on the teaching of mathematics.

In addition, it contains the topics that are usually taught in initial and intermediate courses of number theory at the university level, as a basis for a solid academic training. Its main objective is to present the contents in a rigorous and attractive way. Here about 300 examples are developed and more than 500 exercises are proposed, the vast majority with solutions.

In this third edition new examples and exercises have been incorporated, and special care has been taken in the exposition of the topics, so that it is as explanatory as possible; even the themes can be developed in a self-taught way. A large number of solutions to the proposed exercises have also been included; in others, the final answer or a suggestion to guide solutions. In this new edition, the reading public will find greater freshness and fluidity in the presentation of the topics.

The Technological Publishing House of Costa Rica publishes this winning work of an honorable mention in the Jorge Volio Award 2008, with the certainty that it represents a valuable contribution for those who want to assimilate knowledge, delve into this wonderful branch of mathematics and enjoy the joy of discovery.