Can a Limit Converging to the Square Root of x be Proven from Given Statements?

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SUMMARY

The discussion centers on proving that the limit of a sequence \( a_n \) converges to \( \sqrt{x} \) given the conditions \( a_n^2 \ge x \), \( a_{n+1} \le a_n \), \( x > 0 \), and \( \inf a_n > 0 \). Participants concluded that the provided conditions are insufficient to guarantee convergence to \( \sqrt{x} \), as a counterexample exists where a decreasing sequence converges to \( \sqrt{x + 1} \) instead. Thus, the claim that \( \lim_{n \to \infty} a_n = \sqrt{x} \) cannot be established as universally true.

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  • Knowledge of the concept of infimum in real analysis
  • Basic understanding of square roots and inequalities
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3102
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Homework Statement


I have given the statements: ##a_{n}^2 \ge x## , ##a_{n+1} \le a_{n}## , ##x > 0## and ##\inf a_{n} > 0 ##. How to prove the following: ##\lim_{n \to \infty}a_{n}=\sqrt{x}##

Homework Equations


##a_{n}^2 \ge x## , ##a_{n+1} \le a_{n}## , ##x > 0## and ##\inf a_{n} > 0 ##
##\lim_{n \to \infty}a_{n}=\sqrt{x}##

The Attempt at a Solution


I have come so far: $$a_{n}\ge a_{n+1} \ge \sqrt{x}$$ How shall I continue?
 
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3102 said:

Homework Statement


I have given the statements: ##a_{n}^2 \ge x## , ##a_{n+1} \le a_{n}## , ##x > 0## and ##\inf a_{n} > 0 ##. How to prove the following: ##\lim_{n \to \infty}a_{n}=\sqrt{x}##

Homework Equations


##a_{n}^2 \ge x## , ##a_{n+1} \le a_{n}## , ##x > 0## and ##\inf a_{n} > 0 ##
##\lim_{n \to \infty}a_{n}=\sqrt{x}##

The Attempt at a Solution


I have come so far: $$a_{n}\ge a_{n+1} \ge \sqrt{x}$$ How shall I continue?

Are you sure that's the problem? There's not enough information there to show that the limit is ##\sqrt{x}##
 
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You can't prove this, it is not true!

For example, a decreasing sequence that converges to \sqrt{x+ 1} will satisfy all the hypotheses of your "theorem".
 
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