A function φ from a group (G, ∗) to a group (H, ∘) is operation preserving (a homomorphism) if φ(a ∗ b) = φ(a) ∘ φ(b) for all a and b in G. To prove it, take general elements and compare both sides.
| Function | Working | Operation preserving? |
|---|
| φ: (ℤ, +) → (ℤₙ, +ₙ), φ(x) = x mod n | φ(a + b) = (a + b) mod n = ((a mod n) + (b mod n)) mod n = φ(a) +ₙ φ(b) | Yes |
| φ: (ℝ, +) → (ℝ⁺, ×), φ(x) = eˣ | φ(a + b) = eᵃ⁺ᵇ = eᵃ · eᵇ = φ(a) · φ(b) | Yes |
| φ: (ℤ, +) → (ℤ, +), φ(x) = x + 1 | φ(a + b) = a + b + 1, but φ(a) + φ(b) = a + b + 2 | No |
| φ: (ℤ, +) → (ℤ, +), φ(x) = 3x | φ(a + b) = 3a + 3b = φ(a) + φ(b) | Yes |
When your assignment uses an assigned scenario, write the two operations explicitly first; most lost marks come from mixing up the operation in the domain with the one in the codomain. A homomorphism also sends the identity to the identity and inverses to inverses, which you can quote once the property is proved.