1. Evaluate the following Legendre symbols: (i) (ii) р 463 541 (iii) 8933 104729 (-).

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter2: The Integers
Section2.5: Congruence Of Integers
Problem 58E: a. Prove that 10n(1)n(mod11) for every positive integer n. b. Prove that a positive integer z is...
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[Number Theory] How do you solve question 1? The second picture is for definitions. 

1. Evaluate the following Legendre symbols:
2. Use Gauss' Lemma,
(i)
to compute
(ii)
(iii)
In (ii) p is an odd prime, p ‡ 7; in (iii) p is an odd prime p ‡ 3. In (ii) express the answer
in terms of a congruence condition on p mod 28, and in (iii) express the answer in terms of
a congruence condition on p mod 24, analogous to
= 1 if p = 1 mod 4 and -1 if p = 3 mod 4.
463
541
= (-1)",
4. For an odd prime p, suppose that
31
8933
104729
3. For an odd prime p≥ 7, show that there are consecutive quadratic residues mod p, i.e.,
that there exists an element a € Up such that
μ = |aPN,
a
(²) - ( ² + ¹) -
=
Hint: First show that at least one of the numbers 2, 5 and 10 is a quadratic residue mod p.
x =
a = 1 and consider the congruence
= 1.
=
x² = a mod p.
(i) For p = 3 + 4k, i.e., for p = 3 mod 4, show that the solutions to the congruence are
tak+1
or x =
1
(ii) For p = 5 + 8k, i.e., for p = 5 mod 8, show that the solutions to the congruence are
x = ±ak+¹,
+2²k+1 k+1
a
Explain how to see which is the correct choice.
(iii) Use the results of (i) and (ii) to find the solutions to the congruences
x² = 23 mod 751
x² = 15 mod 797
Transcribed Image Text:1. Evaluate the following Legendre symbols: 2. Use Gauss' Lemma, (i) to compute (ii) (iii) In (ii) p is an odd prime, p ‡ 7; in (iii) p is an odd prime p ‡ 3. In (ii) express the answer in terms of a congruence condition on p mod 28, and in (iii) express the answer in terms of a congruence condition on p mod 24, analogous to = 1 if p = 1 mod 4 and -1 if p = 3 mod 4. 463 541 = (-1)", 4. For an odd prime p, suppose that 31 8933 104729 3. For an odd prime p≥ 7, show that there are consecutive quadratic residues mod p, i.e., that there exists an element a € Up such that μ = |aPN, a (²) - ( ² + ¹) - = Hint: First show that at least one of the numbers 2, 5 and 10 is a quadratic residue mod p. x = a = 1 and consider the congruence = 1. = x² = a mod p. (i) For p = 3 + 4k, i.e., for p = 3 mod 4, show that the solutions to the congruence are tak+1 or x = 1 (ii) For p = 5 + 8k, i.e., for p = 5 mod 8, show that the solutions to the congruence are x = ±ak+¹, +2²k+1 k+1 a Explain how to see which is the correct choice. (iii) Use the results of (i) and (ii) to find the solutions to the congruences x² = 23 mod 751 x² = 15 mod 797
Example 7.2
If n = 7 then we can take =
9 3 as a primitive root. The powers of g in U7
are g = 3, g² = 2, g³ = 6, gª = 4, g5 = 5 and g6 = 1; of these, the quadratic
residues a = 1,2 and 4 correspond to the even powers of g. Thus Q7 is the
cyclic group of order 3 generated by g² = 2.
7.3 The Legendre symbol
We now consider the problem of determining whether or not a given element
a € Un is a quadratic residue. Unfortunately, Lemma 7.3 is not very effective
here: Un is not always cyclic, and even when it is, it can be difficult to find a
primitive root g and then express a as a power of g (see Chapter 6). We therefore
need more powerful techniques. Quadratic residues are easiest to determine in
the case of prime moduli; the case n = 2 is trivial, so we assume for the time
being that n is an odd prime p. The following piece of notation greatly simplifies
the problem of determining the elements of Qp:
Definition
For an odd prime p, the Legendre symbol of any integer a is
if pla,
if a € Qp,
if a € Up\Qp.
0
Clearly this depends only on the congruence class of a mod (p), so we can
regard it as being defined either on Z or on Zp.
0
9-{
= 1
1
-1
Example 7.3
Let p = 7. Then as in Example 7.2 we have
-1
if a = 0 mod (7),
if a 1,2 or 4 mod (7),
if a = 3,5 or 6 mod (7).
Transcribed Image Text:Example 7.2 If n = 7 then we can take = 9 3 as a primitive root. The powers of g in U7 are g = 3, g² = 2, g³ = 6, gª = 4, g5 = 5 and g6 = 1; of these, the quadratic residues a = 1,2 and 4 correspond to the even powers of g. Thus Q7 is the cyclic group of order 3 generated by g² = 2. 7.3 The Legendre symbol We now consider the problem of determining whether or not a given element a € Un is a quadratic residue. Unfortunately, Lemma 7.3 is not very effective here: Un is not always cyclic, and even when it is, it can be difficult to find a primitive root g and then express a as a power of g (see Chapter 6). We therefore need more powerful techniques. Quadratic residues are easiest to determine in the case of prime moduli; the case n = 2 is trivial, so we assume for the time being that n is an odd prime p. The following piece of notation greatly simplifies the problem of determining the elements of Qp: Definition For an odd prime p, the Legendre symbol of any integer a is if pla, if a € Qp, if a € Up\Qp. 0 Clearly this depends only on the congruence class of a mod (p), so we can regard it as being defined either on Z or on Zp. 0 9-{ = 1 1 -1 Example 7.3 Let p = 7. Then as in Example 7.2 we have -1 if a = 0 mod (7), if a 1,2 or 4 mod (7), if a = 3,5 or 6 mod (7).
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