Good book about differential forms

Differential forms are things that live on manifolds. So, to learn about differential forms, you should really also learn about manifolds. To this end, the best recommendation I can give is Loring Tu's An Introduction to Manifolds. Tu develops the basic theory of manifolds and differential forms and closes with a exposition of de Rham cohomology, which allows one to extract topological information about a manifold from the behavior of the differential forms on it.

A finite set of distinct positive numbers is special if each integer in the set divides the sum of all integers within the set.

Interesting behavior of the $2$-tag system $\{a\rightarrow abc,b\rightarrow ab,c\rightarrow c\}$

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Is there a (deep) relationship between these various applications of the exponential function?

What is the smallest value of $n$ for which $\phi(n) \ne \phi(k)$ for any $k<n,$ where $n=4m$?

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product of six consecutive integers being a perfect square

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Existence of divisors of degree one on a curve over a finite field

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How to prove this inequality $\sin{\left(\frac{\pi}{2}ab\right)}\le\sin{\left(\frac{\pi}{2}a\right )}\sin{\left(\frac{\pi}{2}b\right)}$?