18. Numerical evidence makes it plausible that there are infinitely many primes p such that p+50 is also prime. List 15 of these primes.

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thermore, because p|b and b|a, it follows that p|a. The point is simply this: a
44
ELEMENTARY NUMBER THEORY
consecutive primes in infinitely many ways. For example,
6 = 29- 23 = 137 – 131 = 599 – 593 = 1019 – 1013 = . ..
Express the integer 10 as the difference of two consecutive primes in 15 ways.
15. Prove that a positive integer a > 1 is a square if and only if in the canonical form es
all the exponents of the primes are even integers.
16. An integer is said to be square-free if it is not divisible by the square of any integer greater
than 1. Prove the following:
(a) An integern> 1 is square-free if and only if n can be factored into a product ae
distinct primes.
(b) Every integer n > 1 is the product of a square-free integer and a perfect square.
[Hint: If n = p p
29; +rị where r; = 0 or 1 according as k; is even or odd.]
... pk is the canonical factorization of n, then write k: -
2km, wherek>0 and m is an odd
17. Verify that any integer n can be expressed as n =
integer.
18. Numerical evidence makes it plausible that there are infinitely many primes p such that
p+50 is also prime. List 15 of these primes.
19. A positive integer n is called square-full, or powerful, if p² |n for every prime factor p
of n (there are 992 square-full numbers less than 250,000). If n is square-full, show that
it can be written in the form n = a²b°, with a and b positive integers.
3.2 THE SIEVE OF ERATOSTHENES
Given a particular integer, how can we determine whether it is prime or composite
and, in the latter case, how can we actually find a nontrivial divisor? The most
obvious approach consists of successively dividing the integer in question by each
of the numbers preceding it; if none of them (except 1) serves as a divisor, then the
nteger must be prime. Although this method
e regarded as useful in practice. For even if one is undaunted by large calculations.
ne amount of time and work involved may be prohibitive.
There is a property of composite numbers that allows us to reduce materially
e necessary computations-but still the process remains cumbersome. If an in
ger a > 1 is composite, then it may be written as a =
<c<a. Assuming that b < c, we get b' < bc =
- 1. Theorem 3.2 ensures that b has at least one prime factor p. Then p <b < Jo:
very simple to describe, it cannot
bc, where 1 <b <a and
= a, and so b< Va. Because
mposite number a will always possess a prime divisor p satisfying
In testing the primality of a snecific intau
Transcribed Image Text:thermore, because p|b and b|a, it follows that p|a. The point is simply this: a 44 ELEMENTARY NUMBER THEORY consecutive primes in infinitely many ways. For example, 6 = 29- 23 = 137 – 131 = 599 – 593 = 1019 – 1013 = . .. Express the integer 10 as the difference of two consecutive primes in 15 ways. 15. Prove that a positive integer a > 1 is a square if and only if in the canonical form es all the exponents of the primes are even integers. 16. An integer is said to be square-free if it is not divisible by the square of any integer greater than 1. Prove the following: (a) An integern> 1 is square-free if and only if n can be factored into a product ae distinct primes. (b) Every integer n > 1 is the product of a square-free integer and a perfect square. [Hint: If n = p p 29; +rị where r; = 0 or 1 according as k; is even or odd.] ... pk is the canonical factorization of n, then write k: - 2km, wherek>0 and m is an odd 17. Verify that any integer n can be expressed as n = integer. 18. Numerical evidence makes it plausible that there are infinitely many primes p such that p+50 is also prime. List 15 of these primes. 19. A positive integer n is called square-full, or powerful, if p² |n for every prime factor p of n (there are 992 square-full numbers less than 250,000). If n is square-full, show that it can be written in the form n = a²b°, with a and b positive integers. 3.2 THE SIEVE OF ERATOSTHENES Given a particular integer, how can we determine whether it is prime or composite and, in the latter case, how can we actually find a nontrivial divisor? The most obvious approach consists of successively dividing the integer in question by each of the numbers preceding it; if none of them (except 1) serves as a divisor, then the nteger must be prime. Although this method e regarded as useful in practice. For even if one is undaunted by large calculations. ne amount of time and work involved may be prohibitive. There is a property of composite numbers that allows us to reduce materially e necessary computations-but still the process remains cumbersome. If an in ger a > 1 is composite, then it may be written as a = <c<a. Assuming that b < c, we get b' < bc = - 1. Theorem 3.2 ensures that b has at least one prime factor p. Then p <b < Jo: very simple to describe, it cannot bc, where 1 <b <a and = a, and so b< Va. Because mposite number a will always possess a prime divisor p satisfying In testing the primality of a snecific intau
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