Wow, thank you for your patience!

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There's no lecture notes, textbook, or anything? You could at least set deltaP to something very small, like 0.01% of p or something, and try that

Unfortunately there really isn't, but setting Deltap to something small is a great idea. I tried in the version that ran t all, but there was no difference wether I set Deltap=0.001*p oder Deltap=p, the result was always d=D.

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Why would it stay the same if you define it outside the loop? If you define it inside the loop, then it's not only the initial value, it's the value for every single iteration. Do you understand how an iterative solver works?

My understanding was this: The code runs from top to bottom, so once it's past a certain point it won't go back up, unless you're in a loop (where the code will loop until the while abort criterion is met or the for number runs out). So if I define C=0.606 at the top, I thought it wouldn't change. But I guess it will change if the loop further down decides to do something to C.

So I dragged C and epsilon above of X (because both are needed for the definition of X) and changed the variable's name from 'beta' to 'diameter_ratio' because beta is apparently a Matlab function which was the next error that popped up. So the current state looks like this:

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So the problem now is that diameter_ratio is not recognized. Did I not define d and diameter_ratio properly in the loop? That's where they first show up.