Review of homework:
Section 2.3 #17
Section 2.4 2 8 (was also assigned)
Section 2.5 10(b), 12
2.3 #17:
Show that if n has t divisors, then the product of all the positive divisors of n is nt/2.
Solve by induction.
First consider the case where n has only one prime factor. And prove that the product of the divisors is nt/2.
Then assume that the formula is true where n has k prime factors. Show that it is true when n has k+1 prime factors.
Section 2.4 #2:
Describe the algorithm to do trial division by 2, 3 and the numbers in the sequences 6k-1 and 6k+1, k=1,2,...
2, 3, 5, 7, 11, 13, 17, 19, ...
Section 2.4 #8:
Determine the possible remainders of a perfect square when it is divided by 16.
n = 16q + r with r between 0 and 15
n2 = 162q2 + 32 qr + r2
The first two terms are evenly divisible by 16. So we only need to find the possible remainders or r2 mod 16, where r is between 0 and 15.
02 mod 16 = 0
12 mod 16 = 1
22 mod 16 = 2
32 mod 16 = 3
42 mod 16 = 0
and at this point, the pattern clearly repeats.
So r2 can be 0, 1, 4 or 9
Section 2.5, #10(b)
Prove the following identity:
[a, (b,c)] = ([a,b], [a,c])
See proposition 2.5.10 on pg 40.
You end up needing to prove that:
max(a,min(b,c)) = min(max(a,b),max(a,c))
For any three a, b, and c.
Consider all six cases for the ordering of a, b and c.
a ≤ b ≤ c
a ≤ c ≤ b
b ≤ a ≤ c
b ≤ c ≤ a
c ≤ a ≤ b
c ≤ b ≤ a
Section 2.5 #12
Prove that 6n-1, 6n+1, 6n+2, 6n+3 and 6n+5 are pairwise relatively prime.
Use the Euclidean algorithm:
6n+1 = 1(6n-1)+2
6n-1 = (3n-1)2 +1
2 = 2x1 + 0
Linear Diophantine Equations
Find the integer solutions for x and y
ax+by=m
where a, b and m are integers
There are solutions if and only if (a,b) | m.
If there are solutions, and x0 and y0 is a solution, then
x=x0+(b/(a,b))k and y=y0-(a/(a,b))k
are also solutions.
If a and b have a gcd of 1, then there are infinitely many solutions for any m.
ex: 17x+3y=101 or anything
ex: 39x+51y = 7
gcd is 3 and 3 does not divide 7, so no solutions
ex: 119x-29y=8
use the euclidean algorithm to solve
119=4x29+3
29=9x3+2
3=1x2+1
2=2x1+0
GCD is 1.
3=119-4x29
2=29-9x3 = 29-9x(119-4x29) = -9x119 + 37x29
1=3-2 =119-4x29 - (-9x119+37x29) = 10x119-41x29
thus, we can express the gcd as a linear combo of the two coefficients
since our m is 8, multiply by 8
8 = 80x119 - 328x29
this method allows us to find one solution.