Let
F be a field. Prove that if σ is an isomorphism of F(α1, . . . ,
αn) with itself such that σ(αi) = αi for i = 1, . . . , n, and σ(c)
= c for all c ∈ F, then σ is the identity. Conclude that if E is a
field extension of F and if σ, τ : F(α1, . . . , αn) → E fix F
pointwise and σ(αi) = τ (αi) for all i, then σ = τ .