Question

Let X, Y and Z be sets. Let f : X → Y and g : Y → Z functions. (a) (3 Pts.) Show that if g ◦ f is an injective function, then f is an injective function. (b) (2 Pts.) Find examples of sets X, Y and Z and functions f : X → Y and g : Y → Z such that g ◦ f is injective but g is not injective. (c) (3 Pts.) Show that if g ◦ f is a surjective function, then g is a surjective function. (d) (2 Pts.) Find examples of sets X, Y and Z and functions f : X → Y and g : Y → Z such that g ◦ f is surjective but f is not injective.

Answer #1

8.4: Let f : X → Y and g : Y→ Z be maps. Prove that if
composition g o f is surjective then g is surjective.
8.5: Let f : X → Y and g : Y→ Z be bijections. Prove that if
composition g o f is bijective then f is bijective.
8.6: Let f : X → Y and g : Y→ Z be maps. Prove that if
composition g o f is bijective then f is...

Let f : R → R + be defined by the formula f(x) = 10^2−x . Show
that f is injective and surjective, and find the formula for f −1
(x).
Suppose f : A → B and g : B → A. Prove that if f is injective
and f ◦ g = iB, then g = f −1 .

2. Define a function f : Z → Z × Z by f(x) = (x 2 , −x).
(a) Find f(1), f(−7), and f(0).
(b) Is f injective (one-to-one)? If so, prove it; if not,
disprove with a counterexample.
(c) Is f surjective (onto)? If so, prove it; if not, disprove
with a counterexample.

Let f : A → B and g : B → C. For each statement below either
prove it or construct f, g, A, B, C which show that the statement
is false.
(a) If g ◦ f is surjective, then g is surjective.
(b) If g ◦ f is surjective, then f is surjective.
(c) If g ◦ f is injective, then f and g are injective

Let f:A→B and g:B→C be maps. Prove that if g◦f is a bijection,
then f is injective and g is surjective.*You may not use, without
proof, the result that if g◦f is surjective then g is surjective,
and if g◦f is injective then f is injective. In fact, doing so
would result in circular logic.

Let f: Z -> Z be a function given by f(x) = ⌈x/2⌉ + 5. Prove
that f is surjective (onto).

Let A, B, C be sets and let f : A → B and g : f (A) → C be
one-to-one functions. Prove that their composition g ◦ f , defined
by g ◦ f (x) = g(f (x)), is also one-to-one.

For an abelian group G, let tG = {x E G: x has finite order}
denote its torsion subgroup.
Show that t defines a functor Ab -> Ab if one defines t(f) =
f|tG (f restricted on tG) for every homomorphism f.
If f is injective, then t(f) is injective.
Give an example of a surjective homomorphism f for which t(f)
is not surjective.

Show the following:
a) Let there be Y with the cumulative distribution function
F(y). Let F(Y)=Z. Show that Z~U(0,1) for F(y).
b) Let X~U(0,1), and let Y := -ln(X). Show that Y~exp(1)

Let f : R − {−1} →R be defined by f(x)=2x/(x+1).
(a)Prove that f is injective.
(b)Show that f is not surjective.

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