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I'm looking to write a program that will generate valid sudoku puzzles. Essentially, a sudoku puzzle is just an 81 digit number. (The blanks are zeroes.) I would then like to use these puzzles in an Android app.
Is there any risk to using randomly generated puzzles in an app? If another app has the same puzzle, am I infringing somehow? All computer data is numbers, so theoretically a random number generator could stumble upon a copyrighted work at some point.
This seems a lot like asking: Should I worry that something I write might have already been copyrighted by someone else?
The answer, of course, is that most people just don't worry about it. If you write something of any length yourself, rather than copying it from somewhere else, then the chance that you're going to write exactly the same thing that someone else did is pretty small. And if it ever does become an issue, you'll probably have some supporting evidence to show that you really did create the work yourself -- notes, books that you read on the topic, etc.
I think the same thing applies to your sudoku generator. The number of possible puzzles that your program could generate is huge: one source I found with a quick search says the number is 6.6709 * 10^21. Now, how many puzzles could have been copyrighted already? Millions, perhaps? Any way you look at it, the probability that someone playing your game happens upon a copyrighted puzzle must be vanishingly small. Also, the work that you are publishing is not a collection of canned puzzles -- it's just an algorithm that generates puzzles. Nobody can say that you copied their work (unless you lifted the code for generating puzzles from someone else).
TL/DR: Don't worry about it.
Also: I'm not a lawyer, and I'm not giving you legal advice here. I'm just pointing out the odds. If you have legal questions or concerns, talk to your own lawyer.
IANAL, and you should never use legal advice given to you by a stranger on the internet. If you're at all worried contact a lawyer.
Having said that, I doubt very much that Sudoku is under copyright or that it might be possible to copyright an individual puzzle. What is copyrightable is the method of generation; your algorithm. Ensure you make your own up rather than using someone else's. Unless, of course, it's published with an open source licence.
P.S. I dispute your characterization of Sodoku as an 81 digit number; it's a number puzzle that uses 81 single digit numbers.
