# Download PDF by Titu Andreescu: 104 number theory problems : from the training of the USA

By Titu Andreescu

ISBN-10: 0817645276

ISBN-13: 9780817645274

ISBN-10: 0817645616

ISBN-13: 9780817645618

This difficult challenge e-book by way of popular US Olympiad coaches, arithmetic academics, and researchers develops a large number of problem-solving abilities had to excel in mathematical contests and in mathematical learn in quantity idea. supplying suggestion and highbrow pride, the issues in the course of the booklet inspire scholars to specific their rules in writing to give an explanation for how they conceive difficulties, what conjectures they make, and what conclusions they succeed in. using particular strategies and techniques, readers will gather a superior figuring out of the elemental thoughts and ideas of quantity theory.

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Extra resources for 104 number theory problems : from the training of the USA IMO team

Sample text

First note that for p = 2, p 2 + k is always composite for all even positive integers k. Next we note that if p > 3, then p 2 ≡ 1 (mod 3). Hence if k is an even positive integer with k ≡ 2 (mod 3), then p 2 + k is composite for all all primes p > 3 ( p 2 + k is greater than 3 and is divisible by 3). Finally, we note that 32 + k ≡ 0 (mod 5) if k ≡ 1 (mod 5). Putting the above arguments together, we conclude that all positive integers k with ⎧ ⎨ k ≡ 0 (mod 2), k ≡ 2 (mod 3), (∗) ⎩ k ≡ 1 (mod 5), satisfy the conditions of the problem.

1. Foundations of Number Theory 21 In exactly the same way, we can show that there are inﬁnitely many primes of the form 6k − 1. We can view congruency as (part of) an arithmetic progression. For example, we can rewrite the last two results as follows: There are inﬁnitely many primes in the arithmetic progression {−1 + ka}∞ k=1 with a = 4 or a = 6. These are the special cases of a famous result of Dirichlet: There are inﬁnitely many primes in any arithmetic progression of integers for which the common difference is relatively prime to the terms.

On the other hand, gcd(4P − 1, pi ) = 1 for all i with 1 ≤ i ≤ k, and so we ﬁnd another prime that is congruent to 3 modulo 4, a contradiction to our assumption. Hence there are inﬁnitely many primes of the form 4k − 1. 1. Foundations of Number Theory 21 In exactly the same way, we can show that there are inﬁnitely many primes of the form 6k − 1. We can view congruency as (part of) an arithmetic progression. For example, we can rewrite the last two results as follows: There are inﬁnitely many primes in the arithmetic progression {−1 + ka}∞ k=1 with a = 4 or a = 6.