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I have an expression

$\sqrt{1-6e^{ik}+e^{2ik}}$

I want to simplify this expression by defining

$f(k) \equiv 1-6e^{ik}+e^{2ik}$

So that my output will be

$\sqrt{f(k)}$

Thanks

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    $\begingroup$ Try (Full)Simplify with the option TransformationFunctions -> {# /. (1 - 6 E^(i k) + 2^(2 i k) -> f[k]) &, Automatic} $\endgroup$ – Coolwater Mar 30 at 18:41
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Try this code which will do what you want.

Sqrt[1 - 6 Exp[I k] + Exp[2 I k]] /. {1 - 6 Exp[I k] + Exp[2 I k] -> f[k]}

There are other ways to do this in Mathematica, but I think this is the simplest.

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