New posts in Rational Numbers

Given that $\cos\left(\dfrac{2\pi m}{n}\right) \in \mathbb{Q}$ prove $\cos\left(\dfrac{2\pi}{n}\right) \in \mathbb{Q}$

Numbers of the kind $0.aaa\ldots =\frac{1}{aaa\ldots a}$

Can any positive real be approximated as $2^m/3^n$ with $(m,n)$ large enough?

Are terminating decimals dense in the reals?

Polynomial bijections from $\mathbb{Q}$ to $\mathbb{Q}$

Is $H_m - H_n$ a surjection onto $\mathbb{Q}^+$?

"Gaps" or "holes" in rational number system

The set of differences of square rationals

Suppose that $x^5$ and $20x+\frac {19}x$ are rational numbers. Then $x$ is also rational

Does there exist a nonzero ring homomorphism from the ring of square rational matrices to the ring of rational numbers?