Section 1.15

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"The proof is so similar to that of Proposition 1.14 that we omit it."

(a) 

        

(b)

therefore, by (a), .

(c)

Applying (a) gives .

(d)  If, for some , we have , then by (c) we have .  Then substitution gives  and we have the following.

Then , and by (a), .