Maximally dense Unit Distance Graphs

You might be misunderstanding the upper bound. It's more like the upper bound is $O(n^{4/3})$. The actual upper bound is $cn^{4/3}$ where $c = 8$. The paper establishing the upper bound can be found here. Thank you @domotorp for your comment which provided this paper which sets constant $c = 8$.

The integral of an elliptic integral: $\int_{0}^{1}\frac{x\mathbf{K}^2\left ( x \right )}{\sqrt{1-x^{2}}}\mathrm{d}x$

Hard Combinatorical Geometric Problem on Intersecting Circles

An interesting integral $\cos(x)\cos(x^2)\cos(x^3)...$

Recurrence problem: find $a_{1000}$ from $a_{0}$

How would you prove $\int^8_0\frac1{\sqrt{x+\frac1{\sqrt{x}}}}dx<4-\frac1{2019}$?

Characterization of $\mathbb{R}^n$?

Does every non-compact Riemann surface embed holomorphically into $\mathbb{C}^2$?

Proof (without use of differential calculus) that $e^{\sqrt{x}}$ is convex on $[1,+\infty)$.

Quotient Space Metric with Nice Equivalence Classes

An asymptotic series for $\small\prod_{k=n}^\infty\operatorname{sinc}\left({2^{-k}}\pi\right),\,n\to\infty$

What is Bordism good for?

Show that $\sum_{n=1}^{\infty} \frac{(-1)^{n}}{\sqrt{n}}\sin(1 + \frac{x}{n})$ converges uniformly on $\mathbb{R}$