MATH0034 Solution
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MATH0034
Section A
1. (a) Find all solutions to the congruence x3 + x + 5 = 0 mod 35. Solution (9 marks- seen similar).
35 = 5 * 7. We will find all solutions mod 5 and 7 and then put them together using the Chinese Remainder Theorem, since 5 and 7 are coprime.
x = 0, 2, 3 mod 5 and x = 1, 3 mod 7. (4 marks)
Thus, we have 6 solutions:
(5 marks)
(b) Let p e N, p = 1 mod p.
x = 3, 8, 10, 15, 17, 22 mod 35.
mod 40. Explain why 2 and 5 cannot be primitive roots
Solution (8 marks- seen similar).
In particular, p = 1 mod 8. Hence 2 is a quadratic residue mod p. By Euler’s
Criterion, 2 2 = 1 mod p, hence 2 cannot be a primitive root mod p since it’s order is ≤ (p - 1)/2 mod p. (5 marks)
Similarly, since p = 1 mod 5, 5 is a quadratic residue mod p so it cannot be a primitive root mod p. (3 marks)
(c) Let ζ22 = e2πi/22. Compute the product
(2 - ζ2(a)2 ).
1 ≤a<22, hcf(a,22)=1
Solution (8 marks- seen similar).
First find the cyclotomic polynomial
x22 - 1 x22 - 1 x11 + 1
Φ 1 (x)Φ2 (x)Φ1 1(x) (x - 1)(x + 1)(x11 - 1)/(x - 1) x + 1 .
5 marks)
The product above is Φ22 (2) = = 683. (3 marks)
2. (a) (i) Compute the Legendre Symbol sented in the lecture notes that
╱ 、. State clearly all the properties pre-
you are using.
Solution (8 marks- seen similar; students should get at most half the marks if they do not explain their steps clearly).
( ) = ( )( ) (since the Legendre Symbol is multiplicative)
= 1 * ( ) (since 97 = 1 mod 4 and 97 = 1 mod 8 ), respectively ) = 1 * 1 * ( ) (by Quadratic Reciprocity, since 97 = 1 mod 4 ) = ( )
= ( )( ) (since the Legendre Symbol is multiplicative)
= (-1) * ( ) (since 13 = -3 mod 8 )
= (-1) * 1 * ( ) ( by Quadratic Reciprocity, since 13 = 1 mod 4) = (-1) * ( )
= (-1) * 1
= -1.
(ii) Show that if x, y e Z satisfy the congruence 26x2 = y2 mod 97, then
26x2 = y2 mod 972 .
Solution (7 marks- not seen).
Since ╱ 、 = -1, 97|x (otherwise, 26 = (x_1y)2 mod 97, which is not
possible since 26 is not a QR mod 97). But then 97|y2 and, since 97 is prime, we get that 97|y. Thus, 972 |26x2 - y2 ÷ 26x2 = y2 mod 972 .
(b) For which odd primes p e N does the congruence x2 = 15 mod p have solu- tions? Your answer should be given in terms of congruences mod 60. Solution (10 marks- seen similar).
╱ 、= ╱ 、╱ 、= ╱ 、╱ 、.
From the lecture notes, we know that ╱ 、 = 1 if p = 士1 mod 12 and ╱ 、 = 1
if p = 士5 mod 12. (2 marks)
By direct computation, ╱ 、 = 1 if p = 士1 mod 5 and ╱ 、 = -1 if p = 士3
mod 5. (4 marks)
Putting the above together using the Chinese Remainder Theorem, we get that 15 is a QR mod p if any only if p = 士1, 士7, 士11, 士17 mod 60. (4 marks)
3. (a) Let f (x) = x7 + 200x4 + x3 + 9x + 32. Remark that f (1) = 0 mod 3. Find a solution to the congruence
f (x) = 0 mod 36 .
Solution (9 marks- seen similar).
f( (x) = 7x6 + 800x3 + 3x2 + 9.
f( (1) = 819 so vp (f( (1)) = 2. (3 marks)
f (1) = 243 = 0 mod 34 , so we can use Hensel’s Lifting Method with c = 2, which means that a1 will be a solution to the congruence mod 36 , where
a1 = a0 - = 1 - mod 36 .
(3 marks)
It is enough to find 91_1 mod 33 . So
(3 marks)
mod 33 . Using Euclid’s Algorithm, we find 91_1 = -8 a1 = 217 mod 36 .
(b) (i) Using the fact that log(1 + 3x) converges 3-adically for all x e Z(3) , solve the congruence
28x = 55 mod 37 .
Solution (9 marks- seen similar). log(1+33 x) = 33 x - x2 = 33 (x -54*x2 ) mod 37 .
Thus log(28) = 33 * (-53) mod 37 and log(55) = 33 * 29 mod 37 . (4 marks)
Taking log’s on both sides of the congruence, we obtain
log(28)x = log(55) mod 37 ,
which, from the calculations above is equivalent to
x = (-53)_1 * 29 = 55 * 29 = 56 mod 34 .
(5 marks)
(ii) Solve the congruence
x28 = 55 mod 37 .
Solution (7 marks- seen similar). The fastest way is to use the binomial series:
551/28 = (1+2*32 )1/28 = 1+ + *(32 *2)2 = 1+ = 784 mod 37
(3 marks)
Since mod 3 the solutions to the congruence are 士1 and one easily checks using Hensel’s Lemma that they lift uniquely to solutions mod 37 , the only solutions to the given congruence are x = 士784. (4 marks) Alternatively:
Clearly, x must be invertible mod 3, so we can decompose it uniquely as x = T (a) exp(b) mod 37 , where T (a) denotes the Teichmuller lift of a mod 37 . Thus, x28 = T (a28 ) exp(28b). (1 mark)
In the previous part, we found that log(55) = 33 * 29 mod 37 and since 55 = 1 mod 3, we have 55 = T (1) exp(33 * 29) mod 37 . (1 marks) By the uniqueness of the decompositions above, we must have a28 = 1 mod 3 and 28b = 33 * 29 mod 37 . (1 marks) The first congruence has solutions a = 士1 mod 3. (1 mark)
The second congruence has solutions if and only b = 33 * b( , for b( e Z coprime to 3 satisfying 28 * b( = 29 mod 34 兮 b( = 28_1 * 29 mod 34 兮 b( = 56 mod 34 (note that the last computation was already done in the previous part.) (2 marks)
Thus, x = 士 exp(56*33 ) mod 37 = 士(1+33 *56+36 * ) mod 37 = 士784 mod 37 . (1 mark)
4. (a) (i) Determine the value of the continued fraction [5; 5, 10].
Solution (7 marks- seen similar). Let α = [5; 5, 10]. Then, α -5 = (4 marks) ÷ α -5 = ÷ α2 = 27 ÷ α = ,27, since α > 0 (3 marks).
(ii) Find two solutions (x, y) in positive integers to Pell’s equation x2 - 27y2 = 1.
Solution (9 marks- seen similar). The first two convergents of the contin- ued fraction of ,27 are , and the second one gives us a solution to
Pell’s equation: (x, y) = (26, 5). (4 marks)
This means that N (26 + 5,27) = N (26 + 15,3) = 1 in Z[,3]. (2 marks) So N ((26 + 5,27)2 ) = N (1351 + 260,27) = 1, which gives us another solution in positive integers to Pell’s equation: (1351, 260). (3 marks)
(b) Find a solution x, y e Z to the diophantine equation
x2 + yx + y2 = 403.
Solution (9 marks- seen similar). 403 = 13 * 31 and both 13 and 31 are split in R = Z[] (which is a Unique Factorization Domain). (4 marks) To find a solution to the given equation, we need to find α, β e R such that N (α) = 13 and N (β) = 31. Take for example α = (2 marks) and β = (2 marks). A solution is (x, y) = (-14, 23). (1 marks)
Section B
1. (a) Let P (x) e Z[x] be a monic polynomial of degree at least 1.
(i) Let M e Z such that P (M) = A 0. Show that Q(x) = A_1 P (M +Ax) e Z[x].
Solution (14 marks- not seen). Let P (x) = xd + cd_1 xd_1 + . . . + c1 x + c0 . Using the Binomial theorem,
P (M+Ax) = (M+Ax) +cdd_1 (M+Ax)d_1 +. . .+c1 (M+Ax)+c0 = P (M) mod A.
Since P (M) = A, we get P (M + Ax) = 0 mod A.
Hence Q(x) = A_1 P (M + Ax) has integer coefficients.
(ii) Use the remark in the previous part to show that there are infinitely many primes dividing the integers
P (1), P (2), . . . , P (n), . . . .
Solution (28 marks- not seen).
Suppose there are only finitely many primes dividing the above list of integers. Let’s call them p1 , p2 , . . . pr . In particular, P (n) 0 for all n e N.
Consider now
T (x) := Q(p1 ...pr x) = A_1 P (M + Ap1 ...pr x),
where P (M) = A 0. From the previous part, we know that T (x) e Z[x]. Furthermore, T (n) = 1 mod p1p2 ...pr , for all n e Z.
Now, since P (x) has degree at least 1, T (x) has degree at least one. Hence, there must exist an N e N for which T (N) 士1 and thus T (N) has a prime divisor p which is not one of the pi ’s, i = 1, 2, . . . r, from our observation above. So, p|P (M + Ap1p2 . . . pr N), which is a contradiction.
Thus, there must be infinitely many primes dividing
P (1), P (2), . . . , P (n), . . . .
(b) Let a e Z, d e N.
(i) Consider the dth cyclotomic polynomial, Φd (x). Show that if p is an odd
prime dividing Φd (a) then either p|d or p = 1 mod d.
Solution (12 marks- seen similar).
Suppose p /|d. Since Φd (x)|(xd - 1), Φd (a) = 0 mod p ÷ ad - 1 = 0 mod p, so a is coprime to p and the order of a mod p is a divisor of d. Now suppose that the order of a mod p is a divisor, d( , of d, with d( < d. Then ad′ - 1 = e2d′ Φe (a) = 0 mod p. So there exists e|d( such that Φe (a) = 0 mod p, and e < d since e|d( and d( < d by assumption. It follows that a is a root of xd - 1 = f 2d Φf (x) mod p with multiplicity at least 2, i.e. that xd - 1 = (x - a)2 * f(x) mod p for some polynomial f(x) e Z[x]. But this is impossible, since after taking derivatives, we would get that a must also
be a root of dxd mod p , but dad /= 0 mod p, since p /|d and p /|a.
(ii) Use part (a) to show that there are infinitely many primes congruent to 1 mod d in Z.
Solution (6 marks- not seen).
Since Φd (x) e Z[x] and Φd (x) is monic, we can use the first part of the question to deduce that there are infinitely many primes dividing
Φd (1), Φd (2), . . . Φd (n), . . . .
From the previous part, we know that any prime diving Φd (n) must either divide d or be congruent to 1 mod d. Since there are only finitely many primes dividing d, there must be infinitely many primes congruent to 1 mod d.
2. (a) Let
αn = 2 + + + .... + .
Show that the 2-adic valuation of αn , v2 (αn ), satisfies the inequality
v2 (αn ) ≥ t1 (t - v2 (t)).
Solution(14 marks- not seen). Note that α := log(1 - 2) = - converges 2-adically.
Furthermore, 2α = 2 log2 (1 - 2) = 2 log(-1) = log(1) = 0, hence α = 0. Thus v2 (αn ) = v2 ( n+1 ) ≥ mint≥n+1(t - v2 (t)).
(b) Show that the value of the convergent series
1
23k
k=1
is an irrational number.
Solution (26 marks- not seen).
Let Sn = k(n)=1 . Clearly Sn can be written in reduced form with denomi- nator 23n. We will show that the value of the series, α, satisfies the inequality |α - Sn | < 2*22(1)¥3n for all n e N. From lectures, we know that α must be irrational.
) α -Sn = k=n+1 |
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= |
1 1 23n 23n+i _3n i=1 |
< |
1 1 23n 22*i*3n i=1 |
= |
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2022-04-07