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Sum pattern

  • To: mathgroup at smc.vnet.net
  • Subject: [mg128268] Sum pattern
  • From: "Dave Snead" <dsnead6 at charter.net>
  • Date: Wed, 3 Oct 2012 03:07:29 -0400 (EDT)
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Hi,

I'm trying to put together a rule whose left hand side is a sum of arbitrary 
length whose elements all have the same head f.

For example:

In[4]:= x = f[a1, s] + f[a2, s] + f[a3, s]

Out[4]= f[a1, s] + f[a2, s] + f[a3, s]

In[6]:= y = f[First /@ x, s]

Out[6]= f[a1 + a2 + a3, s]

which is what I want.

However when I turn this into a rule

In[7]:= z = x /. (p : Plus[__f]) -> f[First /@ p, s]

Out[7]= f[f[a1, s], s] + f[f[a2, s], s] + f[f[a3, s], s]

Why isn't z equal to y?
How can I make this rule work?

Thanks in advance,
Dave Snead 





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