Could anyone help me find the real and imaginary parts of this
$$ \frac {z}{(1-e^{z})^{2}} $$
where $z$ is complex? I can brute force it out but I'm worried that I'm missing an easier way, as I will be partially differentiating the two parts.
complex numberscomplex-analysis
Could anyone help me find the real and imaginary parts of this
$$ \frac {z}{(1-e^{z})^{2}} $$
where $z$ is complex? I can brute force it out but I'm worried that I'm missing an easier way, as I will be partially differentiating the two parts.
Best Answer
Multiply the numerator and denominator of the expression by the complex conjugate of the denominator, with the intent to make the denominator real: $$ \frac{z}{(1-\mathrm{e}^z)^2} = \frac{z}{(1-\mathrm{e}^z)^2} \frac{(1-\mathrm{e}^{z^\ast})^2}{(1-\mathrm{e}^{z^\ast})^2} $$ Now, with $z = x + i y$: $$ (1-\mathrm{e}^z)^2 (1-\mathrm{e}^{z^\ast})^2 = ( 1 - 2 \operatorname{Re}(\mathrm{e}^z) + \mathrm{e}^{2 \operatorname{Re} z } )^2 = ( 1 - 2 \mathrm{e}^x \cos y + \mathrm{e}^{2 x})^2 $$ Using $\operatorname{Re}(a b^\ast) = \operatorname{Re}(a) \operatorname{Re}(b^\ast) - \operatorname{Im}(a) \operatorname{Im}(b^\ast) = \operatorname{Re}(a) \operatorname{Re}(b) + \operatorname{Im}(a) \operatorname{Im}(b) $: $$ \begin{eqnarray} \operatorname{Re}( z (1-\mathrm{e}^{z^\ast})^2 ) &=& x ( 1 - 2 \mathrm{e}^x \cos y + \mathrm{e}^{2x} \cos( 2 y)) + y ( 2 \mathrm{e}^x \sin y + \mathrm{e}^{2x} \sin( 2 y) ) \\ & = & x - 2 \mathrm{e}^x \left( x \cos y - y \sin y \right) + \mathrm{e}^{2x} \left( x \cos (2 y) + y \sin(2 y) \right) \end{eqnarray} $$ The final result is the quotient of these two: $$ \operatorname{Re}\left( \frac{z}{(1-\mathrm{e}^z)^2} \right) = \frac{x - 2 \mathrm{e}^x \left( x \cos y - y \sin y \right) + \mathrm{e}^{2x} \left( x \cos (2 y) + y \sin(2 y) \right)}{ ( 1 - 2 \mathrm{e}^x \cos y + \mathrm{e}^{2 x})^2 } $$