Typesetting the mathematical expression "const." How do I write in the right way, that something is constant? I think, usually one does write `= const.`, which should be an abbreviation for the Latin `constat`. Should I write the expression italic ($\textit{const.}$) or upright ($\textrm{const.}$)? As far as I know, fixed mathematical operators should be written upright, but this isn't an operator...

Formally speaking, the right way to doing it would be to write something like $$\int 2x\, \mathrm{d}x = x^2 + k, \ \text{where } k \text{ is constant}$$ If you did insist on doing it that way, to me it would look weird if it was italicised: italicised letters inside equations or formulae are usually interpreted as variables, so you _technically_ end up with some ambiguity if any of the letters $c,o,n,s,t$ are used as variables. So I'd certainly prefer $$\int 2x\, \mathrm{d}x = x^2 + \text{const.}$$ ...but, of course, there is no Mathematical Institute of the Universe to govern this kind of thing, so as they say on Jersey Shore, you do you.