<I was looking for a quick way to combine cats and mathematics this morning, and came across the book Calculus for Cats.

This is a book for people about to take calculus, and for survivors of calculus who still wonder what it was all about. It gently explains the basic concepts and vocabulary without making the reader ever do a single problem.

Basically, the book draws (quite literally) an analogy between the fluid motion of cats at play (or in pursuit of “prey”) and the concepts and techniques of calculus, which focuses on continuous functions.

We at CatSynth remember calculus fondly as a mathematical pursuit. But number theory is more my thing. Calculus primary concerns itself with continuous functions of real and complex numbers, while number theory deals with *discrete* entities, like integers. But in mathematics, all things are interconnected. For example, we demonstrated the connection between the gamma function, pi and factorials, combining continuous and discrete concepts.

Consider the function π(*x*), the prime-counting function. It's a bit unfortunate they chose the symbol π, but it is what it is. Basically, this function counts the number of primes less than or equal to a particular number. For example π(20) would be all the prime numbers less than 20: 2, 3, 5, 7, 11, 13, 17 and 19. So π(20) = 8.

So to calculate π(1000) would one have to literally count all the prime numbers less than 1000, including figuring out which numbers are prime? And what about π(1000000)? Unfortunately, the answer is *yes*. But there are good ways to approximate the number of primes, using the results of the Prime Number Theorem. Those interested in the formal theorem are encouraged to follow the link, but we will skip ahead to one of the interesting results. One of basic functions to come out of calculus is the natural logarithm ln(*x*), whose base is the famous constant *e*. If you don't know about it, go look it up. Otherwise, the rest of this article will not make much sense. One can use ln(*x*) to build more complicated functions in calculus, one of which is the offset logarithmic integral, or Li(*x*):

This is one of those functions, like the gamma function, that cannot be expressed without the use of calculus. Turns out, however, that it is a good approximately for π(*x*), which is very much a discrete concept and quite distant from the continuous motions involved in calculus. The prime number theorem provides the connection.

This article is included in Carnival of Mathematics #31 at **recursivity**.