Homepage Solution manuals Ivan Niven An Introduction to the Theory of Numbers Exercise 7.5.3 (Every convergent is a good approximation)

Exercise 7.5.3 (Every convergent is a good approximation)

Say that a rational number a b with b > 0 is a “good approximation” to the irrational number ξ if

| ξb a | = min ( x , y ) 2 , 0 < y b | ξy x | ,

where, as indicated, the minimum on the right is to be taken over all pairs of integers x , y satisfying 0 < y b . Prove that every convergent h n k n to ξ with n > 0 is a “good approximation”.

Answers

Proof. By the first assertion in Theorem 7.13, if | ξy x | < | ξ k n h n | , where n > 0 , ( x , y ) 2 and y > 0 , then y > k n . Hence, if n > 0 ,

| ξ k n h n | = min ( x , y ) 2 , 0 < y k n | ξy x | .

So every convergent h n k n to ξ with n > 0 is a “good approximation” to ξ . □

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2025-07-31 11:01
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