Mathematics in Economics (Econ SE):

What are some important mathematics results that were first developed in Economics?

What are the most creative uses of Mathematics in Economics?

Uses of convex analysis in Economics

Interesting Math Questions (Math SE and MathOverflow)

A function with positive 𝑛-th derivative has at most 𝑛 roots – an inequality version of the Fundamental theorem of Algebra.

How many strict local minima a quartic polynomial in two variables might have?

Can a cubic polynomial in two real variables have three saddle points?

How many saddle points can a quartic polynomial in two real variables have? All 9?

Math serources for students (Econ SE):

Where to start studying the maths in economics?

What are good advanced textbooks to learn mathematics for economist?

Interesting Economics questions (Econ SE):

Seminal papers that later were proven to contain errors