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Uses for Identity[expr]?

To: mathgroup@smc.vnet.net
Subject: [mg11220] Uses for Identity[expr]?
From: Ersek_Ted%PAX1A@mr.nawcad.navy.mil
Date: Mon, 2 Mar 1998 23:11:09 -0500


I have often wondered what  Identity[expr]  is good for.

In Mathematica for Scientists and Engineers Richard Bahder says, "An
operator that is the Identity plus a derivative can  be represented as
a pure function".  Then he gives the following example:

In[1]:=
d=Identity[#]+D[#,x]&;


Well, we can just skip Identity and get the same result (see below).
In[2]:=
dpx=#+D[#,x]&;
d[f[x]]===dpx[f[x]]

Out[2]=
True

There must be a good reason for having this function. I just can't see
what it is.  Can someone explain?

Ted Ersek

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