What is the notation for differentiating with respect to a variable "d" This might be a trivial question, how I wasn't able to get a good answer. I have a large body of work where I use a variable with the symbol d. If I want to show the derivative of another quantity Y(d) with respect to d, can I still use dY/dd ? I have never come across dd like this.
 A: True, $dd$ kind of looks funny. 
There are 25 other letters available in the lower-case English alphabet. Then there is upper case, Greek, and so on. 
But if one insists on using $d$ as a variable, and Leibniz notation, there is no useful alternative to $dd$. True, one could switch in general to the fashionable straight d for the first d. But $\textrm{d}d$ perhaps looks even worse. 
A: If you are only using 1 variable, perhaps try, for a function $Y(d)$, $Y'(d)={{\partial Y}\over{\partial d}}$?  Why must $\partial$ be used only for more than one variable? It certainly looks better to me, and if you can live with $\partial d$ as a differential, you can even keep Leibnitz notation. $\int_C\ f(Y,d)(\partial s)=\int_C\ f(Y,d\sqrt{\partial Y^2 +\partial d^2}$ seems to me easy to read and is not likely to cause problems when you want ${{\partial f}\over {\partial Y}}$ or ${{\partial f}\over {\partial d}}$. Moreover, if you have a lot of d's, a global search and replace to change dd to $\partial d$ can let the computer do the work rather than you.
