$f^{-1}(U)$ is regular open set in $X$ for regular open set $U$ in $Y$, whenever $f$ is continuous.

Proving that $\sum\limits_{k=0}^{n} {{m+k} \choose{m}} = { m+n+1 \choose m+1 }$

Is $2^{218!} +1$ prime?

What's the point of a Horseshoe map?

Is there a short proof for the Intermediate Value Theorem

Showing an indentity with a cyclic sum

Counting non-isomorphic relations

Orientability of projective space

Generalizing Ramanujan's proof of Bertrand's Postulate: Can Ramanujan's approach be used to show a prime between $4x$ and $5x$ for $x \ge 3$

How do i prove that $\det(tI-A)$ is a polynomial?

When is the closure of an intersection equal to the intersection of closures?

Show that $ e^{A+B}=e^A e^B$

Pattern to last three digits of power of $3$?

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