Prove that "injective function $f:X\to Y$ exists" and "surjective function $g:Y\to X$ exists" is logically equivalent.

What's wrong in my calculation of $\int_0^{3 \pi/4} \frac{\cos x}{1 + \cos x}dx$?

Find $p$ and $q$ such that $x^2+px+q<x$ iff $x \in (1,5)$

Inductive proof of gcd Bezout identity (from Apostol: Math, Analysis 2ed)

Which threshold maximizes the expected size of the final sample?

Euler's phi function.

On sums such as $\sum_{k=0}^\infty \binom{2k}{k}\frac{1}{8^k}=\sqrt{2}$

Oriented knots, equivalence relation

+ C in integration by parts allows for major differences in answers?

Is the set of totally ordered sets totally ordered?

Is $\sqrt{x}$ continuous at $0$?

AMC 2011 Coloring Problem

Define the triple factorial, $n!!!$, as a continuous function for $n \in \mathbb{C}$

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