[Date Index] [Thread Index] [Author Index]

Incoherent value for partial derivative

• To: mathgroup at smc.vnet.net
• Subject: [mg91050] Incoherent value for partial derivative
• From: Miguel <misvrne at gmail.com>
• Date: Mon, 4 Aug 2008 03:26:32 -0400 (EDT)

Let

In[1]: g[x_]:=3x^2+5x;
FullForm[g'[x]]

Out[1]: Plus[5,Times[6,x]]

In[2]: g'[2]
Out[2]: 17

Mathematica works fine and the result is correct. First, it executes
the derivation and then the delayed substitution/assignation. But for
partial derivative Mathematica works different way (not correspondig
to FullForm

In[3]: f[x_,y_]:=x^2+x y^2;
FullForm[\!\(
\*SubscriptBox[\(\[PartialD]\), \(x\)]\ \(f[x, y]\)\)]

Out[3]: Plus[Times[2,x],Power[y,2]]

In[4]: \!\(
\*SubscriptBox[\(\[PartialD]\), \(x\)]\ \(f[1, 2]\)\)
Out[4]: 0

I dont understand the reason.