Question 1: Define a relation ∼ on R2 by stating that (a, b) ∼ (c, d) if and only if a2 + b2 ≤ c2 + d2. Show that ∼ is reflexive and transitive but not symmetric.
- Reflexive
Let (a, b) ∈ R2.
Then, a2 + b2 ≤ a2 + b2
That is (a, b) ∼ (a, b)
Thus, ∼ is reflexive.
- Transitive
Let (a, b), (c, d), (e, f) ∈ R2
Suppose (a, b) ∼ (c, d) and (c, d) ∼ (e, f).
Then, a2 + b2 ≤ c2 + d2 and c2 + d2 ≤ e2 + f2
That is a2 + b2 ≤ c2 + d2 ≤ e2 + f2
In particular, a2 + b2 ≤ e2 + f2
Then, (a, b) ∼ (e, f).
Thus, ∼ is transitive.
- Not Symmetric
Counterexample: Pick (1,1), (2,2) ∈ R2
(1,1) ∼ (2,2) is true since 12 + 12 ≤ 22 + 22
but (2,2) ∼ (1,1) is false since 22 + 22 ? 12 + 12
Question 2: Show that an m x n matrix give rise to a well-defined map from Rn to Rm
We can define a mapping: Rn → Rm
Question 3: Find the error in the following argument by providing a counterexample. "The reflexive property is redundant to the axioms for an equivalence relation. If x ∼ y, then y ∼ x by the symmetric property. Using the transitive property, we can deduce that x ∼ x."
Let R be the relation on the set A = {a, b, c} defined by R = {(a, a), (b, b), (a, b), (b, a)}
This is not an equivalence relation because it is not reflexive ((c, c) ∉ R) even though R is symmetric and transitive.
Since c is not related to some element say x ∈ A satisfying (c, x) ∈ R. Then, we are not allowed to use the argument cRx, xRc ⇒ cRc; since x does not exist.
Let σ = (1 3 5 7) and τ = (2 7 3) be elements of S7
a) Find all elements of the subgroups <σ> and <τ>.
b) What order does σ have? What order does τ have? Can you generalize this to a statement for any cycle? σ is order 4 while τ because orders of permutations are determined by least common multiple of the lengths of the cycles.
c) Compute the product στ. What is its order?
στ = (1 2 7)(3 5), order 6.
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