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MathGroup Archive 2007

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Re: Order of terms

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77743] Re: Order of terms
  • From: dh <dh at metrohm.ch>
  • Date: Fri, 15 Jun 2007 06:21:14 -0400 (EDT)
  • References: <f4r3ic$980$1@smc.vnet.net>


Hi Thomas,

Plus orders its agruments automatically. Therefore, you must either 

replace Plus by e.g. List or redefine PLus what is pretty dangerous.

Here is a rule that replaces by List and orders according to sum of indices:

rule={Plus->List,_[x1:__,x2:Subscript[a,k1_]Subscript[b,k2_],x3:Subscript[a,k3_]Subscript[b,k4_],x4__]/;(k1+k2)>(k3+k4):> 

{x1,x3,x2,x4}};

c[3]//.rule

hope this helps, Daniel



Thomas Madden wrote:

> Given

> 

> 

> In[1]:= c[p_] := Sum[Subscript[a, k]* Subscript[b, n - k], {n, 0, p}, {k, 0,

> n}]

> 

> 

> 

> How can I get the output in the same order that the double sum is written?

> For example I would like to have the output from c[2] be

> 

> 

> 

> In[3]:= Hold[Subscript[a, 0]Subscript[b, 0]+Subscript[a, 0]Subscript[b,

> 1]+Subscript[a, 1]Subscript[b, 0]+Subscript[a, 0]Subscript[b,

> 2]+Subscript[a, 1]Subscript[b, 1]+Subscript[a, 2]Subscript[b, 0]]

> 

> 

> rather than the default

> 

> 

> c[2]

> 

> Thanks in advance for any and all replies.

> 

> Thomas

> 




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