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I am trying to write a command to collapse the sum of terms into an expression with summation. Something like

F_[1, j___] + F_[2, j___] + F_[3, j___] := Sum[F[i, j], {i, 1, 3}]

which is designed to to collapse the LHS, which is a function of 2 or more parameters (for example - F[i,j] or F[i,j,k] - this is the reason for the i___ in the definition) into a summation on the first variable. but I get the following error:

SetDelayed::write: "Tag Plus in F_[1,i___]+F_[2,i___]+F_[3,i___] is Protected"

It seems that I am overriding the Plus definition. I don't know then how to define this properly. Thanks.

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Do you want this to work for any head (hence F_) or only certain symbols? –  mfvonh Jul 19 at 21:08
In addition to the question @mfvonh asked, you should also specify what to do with cases like F[1,i]+F[2,i]+F[3,i] where i would appear as the summation index and as an external variable. That doesn't look like a replacement I would want to make in general. The answer would probably use UpSetDelayed, not unprotecting Plus. –  Jens Jul 19 at 22:09
Related: (20540) –  Mr.Wizard Jul 20 at 7:03

2 Answers 2

Plus is protected. but you can use unprotect.

F_[1, j___] + F_[2, j___] + F_[3, j___] := 
 Defer@Sum[F[i, j], {i, 1, 3}]
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Will that change the definition of "Plus" forever? –  Jerald Jul 19 at 21:49
Just it's effect in after run this code –  Junho Lee Jul 20 at 4:28

I think you may find value in this rule:

sequentialQ[x_List] := x == Range[x[[1]], x[[-1]]]

rule =
  HoldPattern[Plus[s : F_[_, j___] ..]] /; 
    sequentialQ @ {s}[[All, 1]] :>
      (HoldForm[Sum[F[i, j], {i, ##}]] & @@ {Min@#, Max@#} &[{s}[[All, 1]]]);


F[1.0, q, r] + F[1.5, q, r] + F[2, q, r] + F[2.7, q, r] + F[3.0, q, r] /. rule

enter image description here

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