I want to get the Real part of this expression - shouldn't be too hard to evaluate. Why is Mathematica not evaluating but returning the same code?

In == Re[χ/(1 + I ω τ)]
Out == Re[χ/(1 + I ω τ)]

Thank you!

  • $\begingroup$ To directly answer the question as asked: Mathematica has no way of knowing that you intend the symbolic entities $\chi$, $\omega$, and $\tau$ to be real; without further information, such entities might be complex. $\endgroup$
    – murray
    Dec 7, 2014 at 17:50
  • $\begingroup$ Same as this post $\endgroup$
    – SquareOne
    Dec 7, 2014 at 17:58

1 Answer 1


Look at the documentation for Re, under Possible Issues

Re can stay unevaluated for numeric arguments: {Re[Log[2 + I]], Re[Sqrt[1 + I]]}

To get around this, try using Re[ComplexExpand[χ/(1 + I ω τ)]].

  • 3
    $\begingroup$ To go one step further, you can display the real part by assuming the variables are real (just like ComplexExpand did): Refine[Re[ComplexExpand[\[Chi]/(1+I \[Omega] \[Tau])]],{\[Chi],\[Omega],\[Tau]}\[Element]Reals] $\endgroup$
    – seismatica
    Dec 7, 2014 at 8:02
  • $\begingroup$ Sorry, I accidentally downvoted and didn't notice fast enough. Reversed :-) $\endgroup$
    – Yves Klett
    Dec 7, 2014 at 20:06

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