# Highly Cited Mathematical Theorems

# Highly Cited Mathematical Theorems

### These theorems are used heavily in projects that use artificial intelligence, machine learning, and more. Come get your math on!

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Some theorems are cited far more often than others. These are not the most striking theorems, not the most advanced or most elegant, but ones that are extraordinarily useful.

I first noticed this when taking complex analysis where the **Cauchy integral formula** comes up over and over. When I first saw the formula, I thought it was surprising, but certainly didn't think, *I bet we're going to use this all the time. *The Cauchy integral formula was discovered after many of the results that textbooks now prove using it. Mathematicians realized over time that they could organize a class in complex variables more efficiently by proving the Cauchy integral formula as early as possible, then use it to prove much of the rest of the syllabus.

In functional analysis, it's the **Hahn-Banach theorem**. This initially unimpressive theorem turns out to be the workhorse of functional analysis. Reading through a book on functional analysis, you'll see "by the Hahn-Banach theorem..." so often that you start to think, *Really, that again? What does it have to do here?*

In category theory, it's the **Yoneda lemma**. The most common four-word phrase in category theory must be "by the Yoneda lemma..." Not only is it the *most* cited theorem in category theory but it may also be the *only* highly cited theorem in category theory.

The most cited theorem in machine learning is probably **Bayes' theorem**, but I'm not sure Bayes' theorem looms as large in ML the previous theorems do in their fields.

Every area of math has theorems that come up more often than others, such as the **central limit theorem** in probability and the **dominated convergence theorem** in real analysis — but I can't think of any theorems that come up as frequently as Hahn-Banach and Yoneda do in their areas.

As with people, there are theorems that attract attention and theorems that get the job done. These categories may overlap, but often they don't.

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