# Derive a symbolic expression

Sorry, I am new to Mathematica and this problem is doing my head in. I have the code in the picture below, I would expect Mathematica to do the derivative wrt q_1 when I write U'[q_1]. However, it just rewrites the command.

I would appreciate it if sb could point out what I am doing wrong and how I could solve it.

KR

• U’ denotes the derivative of U and U’[q] denotes the derivative value at q. However q_1 denotes a Pattern [] object with the head 1, which is almost certainly not what you meant. If U’[q] does not evaluate to something else then it’s because there are no definitions for U to be applied Mar 15, 2022 at 20:10
• Welcome to Mathematica StackExchange! For your further questions, please include your code in a copy-pastable form, not as a screenshot. And to answer your question: You have defined U with three arguments. Therefore, there is no such thing as U with one argument in your code. To get the derivative with respect to the first argument, use D[U[q0, q1, q2], q0]. However, as @MichaelE2 already pointed out, you also should avoid using subscripts. Mar 15, 2022 at 20:16
• In Mathematica, the square brackets are reserved for arguments of functions. Therefore, placing a subexpression at the end of your expression into square brackets is a syntactic error. Use the round ones. Mar 15, 2022 at 20:47
• The image was too small for me to see. I think you're asking for Derivative[0, 1, 0][U][q1, q2, q3]. You want the partial derivative, not U', I think. Mar 16, 2022 at 1:28

Clear[U, Subscript[q, 0], Subscript[q, 1], Subscript[q, 2]]
U[a_, b_, c_] :=
Subscript[\[Alpha], 0] a + Subscript[\[Alpha], 1] b +
Subscript[\[Alpha], 2] c + (1/2) (Subscript[\[Beta], 1] b^2 +           Subscript[\[Beta], 2] c^2 + 2 \[Gamma] b c);
Subscript[\[Alpha], 0] = 1
U[Subscript[q, 0], Subscript[q, 1], Subscript[q, 2]]
D[U[Subscript[q, 0], Subscript[q, 1], Subscript[q,
2]], Subscript[q, 1]]


$$\alpha _1 q_1+\alpha _2 q_2+\frac{1}{2} \left(\beta _1 q_1^2+\beta _2 q_2^2+2 \gamma q_2 q_1\right)+q_0$$

$$\alpha _1+\frac{1}{2} \left(2 \beta _1 q_1+2 \gamma q_2\right)$$