ELI5: Explain Like I'm 5

Circulant

Okay kiddo, have you ever played with a pattern that repeats itself over and over again? That's kind of what a circulant is.

Imagine you have a row of numbers, like 1,2,3,4,5. If you shift the row to the left, the number 1 will go to the end of the row and all the other numbers will shift over to fill the gap, so the row will now look like 2,3,4,5,1.

A circulant takes that idea and makes it even more repeating. Instead of just shifting the first number to the end, a circulant takes the first number and puts it at the end, but also takes the second number and puts it in the first spot, and so on until all the numbers have been shifted. So our row of 1,2,3,4,5 would become:

1,2,3,4,5 (original row)
5,1,2,3,4 (shift 1)
4,5,1,2,3 (shift 2)
3,4,5,1,2 (shift 3)
2,3,4,5,1 (shift 4)

See how the pattern just keeps repeating itself? That's what makes it a circulant.

Circulants show up in all kinds of math stuff, like matrices and number theory, but basically they're just a way of describing a repeating pattern. Kind of like how you might draw a picture of a flower and then keep repeating it to make a pretty design. Does that make sense, kiddo?