Thursday, September 6, 2018

real analysis - Proving that if $lim_{n rightarrow infty} a_n = a$ then $lim_{n rightarrow infty} a_n^2 = a^2$




If we have a real sequence $\left|a_n\right|$ such that $\lim_{n \rightarrow \infty} a_n = a$, how do we prove (by an $\epsilon - N$ argument) that $\left|a_n\right|$ such that $\lim_{n \rightarrow \infty} a_{n}^{2} = a^2$?



I know you can use algebra to do to the following:



$$\left|a_n^2 - a^2\right| =\left|(a_n - a)(a_n + a)\right|$$



Where I feel like you can use the implication that $\lim_{n \rightarrow \infty} a_n = a$ to show that $(a_n-a) < a$ or something.



What's the proper way to go about this?


Answer




Hint



A convergent sequence is bounded. So you can also bound $\vert a+a_n\vert$.


No comments:

Post a Comment

analysis - Injection, making bijection

I have injection $f \colon A \rightarrow B$ and I want to get bijection. Can I just resting codomain to $f(A)$? I know that every function i...