I had a girlfriend who was doing her PhD in Physics. I remember one night she and her classmates spent all night working on a problem, that was essentially unsolvable. The next day they go to class and all of them made their best attempt, but no one could complete it. The problem? The professor accidentally used the wrong metric on one of the numbers meaning that they couldn't do the steps to what should have been an easy solution. Now they noticed this, they are a smart group of people, but that's not what the problem was, so they spent hours and hours trying to solve what he gave them without any hope of actually doing it.

In the end, he was like "Oh my bad," and corrected his mistake. The point of the story is that they were able to essentially ask him for the solution and they were able to check what they'd done, and in the end he made the mistake. In situations like this, he should provide the answer if nothing else to show that he didn't make a mistake in the problem set. People are fallible, no matter how brilliant you are.

I had a similar thing happen in high school physics. We were were suppose to figure out where and when a projectile was going to land. The only problem was that it was never going to land—-the initial velocity was too high.

In retrospect I think it was a great lesson for my future career as a data engineer. Doesn’t matter what the source is, any datum can be just plain wrong.

> The only problem was that it was never going to land—-the initial velocity was too high.

As in, it escaped the gravitational field?

Yep.

That professor probably had produced the answer already, using the metric he had initially intended. Having a set of answers doesn't mean you didn't make a mistake in the questions.

But it does clue you into whether the professor used a different set of assumptions than you did.

Presumably you wouldn't see the answers until after you spent all night on the assignment.

There's a story of a prof injecting one error into every test.

I had a industrial engineering teacher that did something similar. Every team had a (fake money) budget for each project and bought materials from him (sole source). He'd randomly cut corners, to keep everyone on their toes. Good lesson for the real world.

On the other hand, struggling all night to solve an un-solvable problem is a valuable learning experience, too. In the real world we struggle all the time with problems that don't have solutions.

I took a quick look through the first set of exercises and a lot of the problems are open-ended and don't have specific answers. Seeing a different answer than yours doesn't tell you much about whether your answer is correct.

Even if you do have a similar proof to the one in the answer key, that doesn't mean your proof is correct. Correctness can often hinge on very subtle distinctions. You really need a TA or instructor to read your proof if you want meaningful feedback.

As I've said elsewhere, I'm looking out for my own students first.

One of my colleagues suggested setting up a "club" on PerusAll (https://app.perusall.com/welcome) or something similar that would allow people to discuss to book and/or work through problems collectively. (They have a club for CLRS, for example.) Right now all their "clubs" are restricted to books with Reputable Publishers (ptui), so they might need some persuasion.

Presumably nothing's stopping you from putting a book of solutions and posting them online. It seems like a much easier task than writing this book, with all its problem sets, and then posting it online for free for the world to read.

It might be an elitist and gate keeping mentality but I have to say that calling providing a free resource to someone, but not tailoring it to fit their exact situation, "robbing" is a very entitled one.