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Maybe the way I expressed is not very accurate, but a very simple example will help to understand the question:

For

Subscript[a,i]+Subscript[a,j]**Subscript[a,k]

I would like to replace a pattern that a_i shows exactly one time with some other symbol.

An intuitive way fails to work using Except:

Subscript[a, i] + Subscript[a, j] ** Subscript[a, k] /. 
 Except[Subscript[a, x_] ** Subscript[a, y_], Subscript[a, z_]] -> zz

The output is zz + zz ** zz. However, I would like it to be zz+Subscript[a, j] ** Subscript[a, k].

The reason I need this is that I want to drop the linear term but keep the quadratic term of a_i by replacing only the linear term of a_i to zero.

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    $\begingroup$ Look at Subscript[a, i] + Subscript[a, j] ** Subscript[a, k] // FullForm. That should explain why the pattern does not match. $\endgroup$ – Rohit Namjoshi Apr 12 at 2:28
  • $\begingroup$ @Rohit Namjoshi, Thanks! I understand why it fails now. But is there a workaround that can realize my idea? $\endgroup$ – Jake Pan Apr 12 at 3:12
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    $\begingroup$ Subscript[a, i] + Subscript[a, j] ** Subscript[a, k] /. Plus[a___, b_Subscript, c___] :> a + zz + c $\endgroup$ – LouisB Apr 12 at 4:23
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You can also do

Subscript[a, i] + Subscript[a, j] ** Subscript[a, k] /.
  {a_NonCommutativeMultiply :> a, Subscript[a, z_] -> zz}
  zz + Subscript[a, j] ** Subscript[a, k]

because ReplaceAll >> Details:

  • "ReplaceAll looks at each part of expr, tries all the rules on it, and then goes on to the next part of expr. The first rule that applies to a particular part is used; no further rules are tried on that part or on any of its subparts.
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