# Evaluate the limit of a subsequence: Compute the limit $\lim_{n\to\infty} \cos\left( \pi \sqrt{4n^2 + 5n + 1} \right)$ Integer $n$ on Mathematica I want to compute the limit $$\lim \limits_{n\to\infty} \cos\left( \pi \sqrt{4n^2 + 5n + 1} \right)$$for integer $$n$$. By completing the square, we can determine that this limit is equal to $$– \tfrac1{\sqrt2} \approx -0.7071$$.

But if we don’t restrict $$n$$ to an integer, then the limit is indeterminate / does not exist. And can be easily found by typing it on WolframAlpha. Or in Mathematica: However, I do not know how to compute the limit (on Mathematica) with the original constraint that $$n$$ must be an integer.

I know that we can plot a graph on Mathematica: The graph suggests that the limit is equal to $$-\tfrac1{\sqrt2}$$. However, this doesn’t look like a convincing result because we can’t know that the limit is exactly equal to $$-\tfrac1{\sqrt2}$$.

Question: Is there a way to compute this limit in Mathematica where it spits out a single numerical value (of $$-1/{\sqrt2}$$)? Posted on Categories cheapest proxies