So I am designing a mobile app that transposes music for the user. The user can only transpose four notes at a time, and they are required to input the notes themselves. Are there any copyright laws of any kind that might cause problems with my app?
Four notes seems an odd limit, and I wonder if this would be useful to anyone. But that is not the question. If a user types in a copyrighted composition note by note and transposes it, the user may well have infringed the copyright by creating a derivative work. But the app designer and proprietor will not have infringed, unless it was advertised with such a use in mind, knowing that infringing uses were likely.