The aim of this course is to introduce a selection of proofs of some important theorems. These proofs require moderate background but high ingenuity. Among the topics are: Division algorithm, prime factorization theorem, some primitive results on the distribution of primes. Greatest common divisor. Euler's totient function. Phytagorean triples. A short survey of metric spaces; continuity, compactness, connectedness. Stone- Weierstrass approximation theorem. Geometry of the sphere. Brouwer fixed point theorem. Borsuk's antipodal mapping theorem.