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Question about Collect


Consider the following simple expression

expr = Collect[x^3 - c*x^2 - 3*a*x - b /. x -> y + c/3, y]
-b - a*c - (2*c^3)/27 + (-3*a - c^2/3)*y + y^3

I can't understand the difference between the two outputs below

expr /. {-b - a*c - (2*c^3)/27 -> A1, -3*a - c^2/3 -> A2}
A1 + (-3*a - c^2/3)*y + y^3

(*and*)

expr /. -b - a*c - (2*c^3)/27 -> A1 /. -3*a - c^2/3 -> A2
A1 + A2*y + y^3

I waited to be the same.

Any guidance/insight?

Dimitris


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