[Math] $\int_{-\infty}^\infty \frac{\sin (t) \, dt}{t^4+1}$ must be zero and it isn’t

complex-analysiscontour-integrationimproper-integralsresidue-calculus

I'm trying to evaluate the integral
$$\int_{-\infty}^\infty \frac{\sin (t) \, dt}{t^4+1}$$ using residue and complex plane integration theory.

Let $f(t):=\frac{\sin (t)}{t^4+1}$, $f(z):= \frac{\sin (z)}{z^4+1}$. Then $f(z)$ has four singular points, two of which are in the semicircle $R>2$ in the upper half of the complex plane: $p_1:=\exp\{i\frac{\pi}{4}\}$ and $p_2:=\exp\{i\frac{3\pi}{4}\}$.

We know that $$\int_{-R}^R \frac{\sin (t) \, dt}{t^4+1}+\int_{C_R}\frac{\sin (z) \, dz}{z^4+1}=2\pi i(\operatorname{Res}_{z=p_1}+\operatorname{Res}_{z=p_2})$$

But here's the mysterious part:
$$\lim_{R\to\infty}\int_{C_R}\frac{\sin (z) \, dz}{z^4+1}=0$$
yet $$(\operatorname{Res}_{z=p_1}+\operatorname{Res}_{z=p_2})\ne 0$$
But the original integral must be equal to zero. I'd appreciate if it could be pointed out what I'm not doing right.

Best Answer

This :$$\lim_{R\to\infty}\int_{C_R}\frac{\sin (z) \, dz}{z^4+1}=0$$ is false because of the sine. (Which is NOT bounded on $\mathbb{C}$) The usual trick is to consider the integral $$\int_{-\infty}^{+\infty} \frac{e^{it}}{t^4+1} dt,$$ to compute it by residue using Jordan's Lemma and then to take the imaginary part to get the integral you want.

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