In Maple, you have the freedom to use almost any letter as a variable, even when that letter is also used to represent a particular mathematical symbol or constant, such as the exponential constant $\ⅇ\,$the differential symbol $\ⅆ$, or the imaginary unit $\mathrm{i}$.  To achieve this flexibility, these characters need to be entered as special symbols.

This warning occurs if you enter something that resembles a differential expression, but use the variable $d$, not the differential symbol $\ⅆ$. In order to enter the differential symbol, use command completion, the palettes, or the diff command, as demonstrated in the examples below.

Example 1

Typing d/dx by hand does not create a differential expression.  Instead, the d is simply a variable (and the same with dx).

Solution 1 - Use the command completion

Use command completion to enter a differential expression.  Start by typing 'diff', then press Esc.  From the pop-up list, select the desired template.

In this case, we select .  It is inserted with the placeholder x selected.  Type x (or whatever your variable is).  Use the right arrow to exit the denominator, then type x^2.

Tip: If you only need the $\ⅆ$ symbol and not the full template, type d and press Esc, and then select Differential 'd' from the pop-up list.

Solution 2 - Use the Calculus palette

The Calculus palette (along with the Expression palette) provide templates for entering differential expressions.

Place your cursor in your document, then click the   template.  Press Tab to go to the first placeholder $x$.  Type x.  Press Tab to go to the second placeholder.  Type x^2.  Press enter to execute.  You can see this is now an expression for the derivative of $x^2$ with respect to $x$.

Note the parentheses around the expression that is being differentiated.  In this case it works with or without the parenthesis, but for another expression, for example the polynomial $x^2-40 x-6$, you must use parentheses:

Solution 3 - The diff command

You can also use the diff command to perform differentiation:

See the diff help page for additional information.