ELI5: Explain Like I'm 5

Motzkin number

Okay kiddo, have you ever counted how many ways you can climb up the stairs in your house? Or how many different paths there are to walk from your bedroom to the kitchen? These are called counting problems, and they're really important in many different areas of math, like combinatorics.

Motzkin numbers are a way to count certain kinds of paths or sequences. Think of them like a special kind of puzzle - you have to figure out how many different ways you can arrange a certain set of pieces or steps.

Imagine you have a set of blocks or tiles, some of which are red, some of which are green, and some of which are blue. You want to arrange these blocks in a sequence or pattern, but there are a few rules you have to follow:

1. You can't have two red blocks in a row.
2. For every green block, there has to be a matching blue block somewhere later in the sequence.

For example, one possible arrangement might be: RGRBG, where R stands for red, G for green, and B for blue. But there are many other possible arrangements that follow these rules.

Motzkin numbers tell you how many different arrangements there are, based on how many blocks you have. For example, if you have just one block, there's only one possible arrangement - it can't have two red blocks in a row or a green block without a matching blue one.

But if you have two blocks, there are three possible arrangements: RG, RB, and BG. And if you have three blocks, there are five possible arrangements: RGRB, RBRG, RBGB, BRGB, and BRGR.

These numbers get bigger and more complex as you add more blocks, but mathematicians have figured out ways to calculate them so we don't have to count each one by hand!

So that's the basic idea of Motzkin numbers - they're a way to count certain kinds of sequences or paths, based on certain rules or constraints. It may seem like a simple puzzle, but it's an important concept in math and can be used in many different fields, like computer science and chemistry. Cool, huh?