Differential operator

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You can do :

sage: m.derivative(x)                                                                                                                                                                                        
[x |--> 3]
[x |--> 4]

If you do not like the mapping you can first "evaluate" to get a symbolic expression instead of a symbolic function:

sage: m(x).derivative(x)                                                                                                                                                                                     
[3]
[4]

Compare

sage: m                                                                                                                                                                                                      
[x |--> 3*x]
[x |--> 4*x]

with :

sage: m(x)                                                                                                                                                                                                   
[3*x]
[4*x]