Derivative of piecewise function returns one more case

Question

I define the following piecewise function:

U[c_,n_,g_,p_]:=Piecewise[{{c^(1-g)/(1-g)-n^(1+p)/(1+p),g!=1}, {Log[c]-n^(1+p)/(1+p),g==1}}]

When I take the derivative

D[U[c,n,g,p],c]

It returns a a third unexpected line.

(In the image, gamma corresponds to g)

What is the meaning of this third line and how can I remove it from the printed output?

Answer 1

Clear["Global`*"]

Instead of using g != 1 use g < 1 || g > 1, then

U[c_, n_, g_, p_] := 
 Piecewise[{{c^(1 - g)/(1 - g) - n^(1 + p)/(1 + p), g < 1 || g > 1}}, 
  Log[c] - n^(1 + p)/(1 + p)]

D[U[c, n, g, p], c] // Simplify

(* c^-g *)

EDIT: Or use Simplify with your original definition

U[c_, n_, g_, p_] := 
 Piecewise[{{c^(1 - g)/(1 - g) - n^(1 + p)/(1 + p), 
    g != 1}, {Log[c] - n^(1 + p)/(1 + p), g == 1}}]

D[U[c, n, g, p], c] // Simplify

(* c^-g *)

Answer 2

It looks like Mathematica insists on having a default condition (with value zero, unless otherwise specifed).

With

ass=D[U[c,n,g,p],c]

You can construct something like

Piecewise[Drop[ass[[1]]], ass[[1, 2, 1]]]

This uses the second terms as the fall through value.