Thursday, 31 March 2016

complex analysis - Compute the Integral via Residue Theorem



My goal is to compute I=+0cosax1+x2dx where a>0.




I=12+cosax1+x2dx=12Re(+eiax1+x2dx).



Let f(z)=eiaz1+z2.



By Residue Theorem, RReiax1+x2dx+γReiaz1+z2dz=2πRes(f,i)=ea2i, where γR denotes the upper semi-circle centered at O with radius R.



As R—>+,



RReiax1+x2dx —> +eiax1+x2dx




Now, I am stuck on how to prove γReiaz1+z2dz goes to 0 as R goes to infinity.



Anyone know how to do it? Many thanks.


Answer



Note that for z=R(cos(t)+isin(t)) with R>1 and t[0,π]
|eiaz1+z2|=eaRsin(t)R211R21.
Hence, as R+,
|γReiaz1+z2dz||γR|R21=πRR210.
P.S. This is a particular case of the Jordan Lemma.



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