I want to do following:

ArcTan[1/Sqrt[-1 + (4 Sqrt[2] x^2)/(5 xc^2)]] /.4 Sqrt[2]/5->a^2

I expect to have

ArcTan[1/Sqrt[-1 + x^2/(a^2 xc^2)]]

Mathematica cannot recognize 4 Sqrt[2]/5 in expression.

  • $\begingroup$ Please provide a short but complete example, not just a fragment. Also explain what output you're expecting and what you actually got. $\endgroup$ – Szabolcs Feb 6 '15 at 18:49
  • $\begingroup$ /.4 is ambiguous, add a space to get /. 4 $\endgroup$ – Manuel --Moe-- G Feb 6 '15 at 19:10
  • 1
    $\begingroup$ I was going to say what @Manuel --Moe-- G noted. I'll add that I for one believe this was a deplorable lexing/parsing decision, made by the wrong person, and retained ever since I have no idea why. The rule everywhere else is that a lexeme is as long as it can be. I will add that this bites me particularly hard because I often need to replace zeros (why that happens I don't know, but it does happen frequently). $\endgroup$ – Daniel Lichtblau Feb 7 '15 at 22:37
Sqrt[5/3] + 1 /. Sqrt[5/3] -> a
(* 1 + a *)

Tanh[Sqrt[5/3] x] /. Sqrt[5/3] ->  a
(* Tanh[a x] *)

Try starting a new Mathematica session

| improve this answer | |
  • $\begingroup$ sorry, I tried to keep it as simple as possible, but it does not cover my problem (see edit) $\endgroup$ – stinglikeabeer Feb 6 '15 at 18:57
  • $\begingroup$ @belarus28 Copy this as is and execute ArcTan[1/Sqrt[-1 + (4 Sqrt[2] x^2)/(5 xc^2)]] /. 4 Sqrt[2]/5 -> a^2 $\endgroup$ – Dr. belisarius Feb 6 '15 at 19:01
  • $\begingroup$ wow! it was not working, and now in the new session it works... magic. thanks. $\endgroup$ – stinglikeabeer Feb 6 '15 at 19:12

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