Section III in a Nutshell
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Section III in a Nutshell This section considers functions from one ring R to another ring S that preserve certain algebraic properties: Consider ϕ : R→ S such that
ϕ(a+ b) = ϕ(a) + ϕ(b) and ϕ(ab) = ϕ(a)ϕ(b),
for all a, b ∈ R. We call ϕ a ring homomorphism. A ring homomorphism always preserves the zero of the ring, additive inverses, unity,
and multiplicative inverses (Theorem 11.1). While a ring homomorphism ϕ : R → S need not be onto, it is onto the image of R in S (ϕ(R)) which is itself a subring of S (Theorem 11.2).