MATH3066 ALGEBRA AND LOGIC Semester 1 Second Assignment 2023
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MATH3066 ALGEBRA AND LOGIC
Semester 1
Second Assignment
2023
This assignment comprises 60 marks and is worth 20% of the overall assessment. It should be completed and uploaded into Canvas before midnight on Friday 19 May 2023. Acknowledge any sources or assistance . This must be your own work. Breaches of academic integrity, including copying solutions, sharing answers and attempts at contract cheating, attract severe penalties .
1. Use the rules of deduction in the Predicate Calculus to find a formal proof for the following sequent (without invoking sequent or theorem introduction):
(3北) ╱G(北) ÷ ╱H(北) A K(北)、、, (A北) ~ K(北) 上 (3北) ~ G(北) (5 marks)
2. (a) Find the error in the following argument. Explain briefly.
1 (1) ╱(3北)G(北)、A ╱(Ay)H(y)、 A
1 (2) (3北)G(北) 1 A E
3 (3) G(a) A
1 (4) (Ay)H(y) 1 A E
1 (5) H(a) 4 A E
1, 3 (6) G(a)A H(a) 3, 5 A I
1 (7) G(a)A H(a) 2, 3, 6 3 E
1 (8) (A北) ╱G(北) A H(北)、 7 A I
(b) Find a model to demonstrate that the following sequent cannot be proved using the Predicate Calculus:
╱(3北)G(北)、A ╱(Ay)H(y)、 上 (A北) ╱G(北) A H(北)、
Explain briefly.
(c) Prove the following sequent using rules of deduction from the Predicate Calculus (without invoking sequent or theorem introduction):
(A北) ╱G(北) A H(北)、 上 ╱(3北)G(北)、A ╱(Ay)H(y)、
3. Consider the following well-formed formulae:
W1 = (Vx)╱E(x, x) A ╱ G(x) v H(x)、、, W2 = (ax) ╱G(x) A H(x)、,
W3 = (ax)(ay)╱ ~ G(x) A ~ G(y) A ~ E(x, y)、,
W4 = (ax)(ay)╱ ~ H(x) A ~ H(y) A ~ E(x, y)、
Prove that any model in which W1 , W2 , W3 and W4 are all true must have at least 5 elements. Find one such model with 5 elements. (8 marks)
4. Recall the division ring of quaternions
H = {a + bi + cj + dk I a, b, c, d, e R}
Put β = 2 + i - j + k . Find γ, δ e H such that
βγ = δβ = 3j - 4k .
Verify any claims.
[Hint: you may use without proof the fact that if α = a + bi + cj + dk e H and α = a - bi - cj - dk . then
αα = a2 + b2 + c2 + d2 .] (6 marks)
5. Let R = {0, 1, x, x + 1, x2 , x2 + 1, x2 + x, x2 + x + 1} be the subset of Z2 [x] consisting of all polynomials of degree at most 2, with usual addition and multiplication of polynomials followed by taking the remainder after dividing by x3 + x2 + 1. Then R is a commutative ring with identity (and you do not need to verify this).
(a) Construct the multiplication table for nonzero elements of R, and explain briefly why R is a field. (To get full credit for this part, it is not necessary to show any calculations.)
(b) Solve the following equation over R for α where
α3 + x2 α + x2 + 1 = 0 .
[Hint: α = 1 is a solution.] (8 marks)
6. An element e in any given ring is called idempotent if e2 = e. For example, 0 and 1 are distinct idempotents in any nontrivial ring with identity.
(a) Verify that if e is an idempotent in a ring with identity, then 1 - e is also an idempotent.
(b) Suppose that e is an idempotent in a commutative ring R with identity. Verify that eR is a subring of R, which then becomes a commutative ring with identity under the operations inherited from R.
(c) Suppose that e is an idempotent in a commutative ring R with identity. Prove that
R eR o (1 - e)R .
[Hint: consider the map a ,→ ╱ea, (1 - e)a、.] (12 marks)
7. In this exercise, we explore a decomposition of Z30 using subrings and the direct product construction.
(a) Find all of the idempotents in Z30 . (If you list them correctly, then you get full credit without working or justification.)
(b) Find nonzero idempotents e, f and g in R = Z30 such that R eR o fR o gR .
Verify any claims.
You may use without proof the fact that if R1 , R2 and R3 are rings then
(R1 o R2 ) o R3 R1 o (R2 o R3 ) R1 o R2 o R3 .
(c) How do you reconcile the result of the previous part with the fact that,
for general reasons,
Z30 Z2 o Z3 o Z5 ?
It is a general fact that if p and q are coprime, then pZpq Zq , and you may quote this fact without proof. It is also a general fact, that doesn’t require proof, that taking direct products is commutative up to isomorphism. (12 marks)
2023-05-24