This is my first time to be an instructor for a class and I have to prepare problem sets for my students. For that purpose, I collect problems from different textbooks. Unfortunately, if I cite the source of each problem, students always find the solution manual of this textbook online and copy/edit the answer from the manual. This is certainly not very instructive. So, I thought I can remove all the citations, hoping that this encourages the students to work on the problems on their own. Is this considered plagiarism ? Is there a better solution? Advice from experienced instructors would be very appreciated.