Question: Change in a analytic expression

Hello, 

I'd like to modify the form of this expression

Code:
den := 1.*omega^4/omega[n]^4+1.-2.*omega^2/omega[n]^2+4.*epsilon^2*omega^2/omega[n]^2



To manage to have this form.

 

For the moment, i didn't manage de modify this form by using the "factor" or "collect" function.

Do you have a idea to solve my problem?

Thank you in advance for your help.

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