ELI5: Explain Like I'm 5

Fredholm integral equation

Alright kiddo, let's talk about Fredholm Integral Equations. Imagine you have a puzzle where you have to find a missing piece, but instead of just looking for that piece, you also have to use information about how that piece fits into the puzzle to find it. That's kind of like what a Fredholm Integral Equation is!

Here's a more specific example: Let's say you're playing with blocks and stacking them up in different ways. You notice that when you stack three blocks on top of each other, the height of the stack is always 3 times the height of one block. But when you stack four blocks, the height is always 4 times the height of one block. You wonder if there's a pattern there.

To figure it out, you could use a Fredholm Integral Equation. You might start by writing an equation that says something like: h(3) = 3b, where h(3) is the height of a stack of 3 blocks and b is the height of one block.

But that's not enough to solve the puzzle - you need more information. So you add another equation that relates the height of a stack of 4 blocks to the height of one block: h(4) = 4b.

Now you have two equations, but they're not enough to solve for b or h. So you use some mathematical wizardry to combine the two equations into one, big equation that looks like this:

h(x) - lambda * b = integral from 3 to 4 of K(x,t) * h(t) dt

Woah, that's a mouthful! But it's just a fancy way of saying that the height of a stack of any number of blocks can be expressed as a combination of the height of one block and some integral (which means a kind of fancy sum) involving a special function K(x,t).

With this equation, you can start to solve the puzzle. You know h(3) and h(4), so you can plug those values into the equation and solve for b and h using some more math tricks.

So that's basically what a Fredholm Integral Equation is - it's a puzzle where you have to use information about how different pieces fit together to find the missing piece. It can be used in all sorts of fields, from engineering to physics to economics. But don't worry if it still seems a little confusing - even grown-ups sometimes have trouble with it!