In the US, high-school linear algebra, geometry and calculus were de rigeur. Three courses of calculus were, at the time I went to university, required for engi

In the US, high-school linear algebra, geometry and calculus were de rigeur. Three courses of calculus were, at the time I went to university, required for engineers, along with differential equations and matrix algebra. However, the pure-mathematics coursework ended up being the most useful: two or more mathematical analysis courses, non-Euclidean geometry, statistics, algorithmic analysis, and mathematical topology. The two take-these-if-you-want-a-future courses I’d recommend are discrete mathematics (which I use every day) and abstract algebra (which I also use every day). There are a lot of good college textbooks for both; mine is long gone, though you may find one that you absolutely love.