This is an Ulam spiral: Take a 2D grid, and starting from any specific square, write down each number starting from 1 in a counter-clockwise spiral. Typically, prime numbers are highlighted, to create an interesting pattern — however in this spiral, circles have been drawn with sizes proportional to the number of divisors the number has.
For example, the number 20 is divisible by 1, 2, 4, 5, 10 and 20, so its circle will be 3 times larger than the circle for the number 7, which is only divisible by 1 and 7. Prime numbers will have the smallest circles.
I found the image on the Wikipedia Article for Elementary Number Theory!
(Source: limefriend, via fyeahchemistry)
