We all know how important proper attribution of ideas is. At the same time, certain things have become basic enough that citing the paper where they were first discussed is overkill: to give an extreme example, if you need to do some differentiation in your math/physics paper, you don't need to go and cite Newton and Leibniz. Now, on occasion students ask me how one can determine if a piece of knowledge is common enough that they can forgo a citation. The rule of thumb I give them is:
If it is something that is explained in a standard first-year undergrad textbook, then anybody who is going to read your papers knows about it and you don't need to provide a citation.
[Here I want to emphasize that I give this to them as a rule of thumb, and I always tell them to ignore it and provide the relevant citation if they think it is necessary to do so in a specific case]
Are there better or alternative ways of drawing the line?