Monday, 30 September 2013

residue calculus - Improper integral computation using complex analysis



pvx(x2+4)(x2+2x+2)dx



I get an answer of π5 but wolframalpha disagrees by a factor of 2 (π10):




http://www.wolframalpha.com/input/?i=integration+%28z%2F%28%28z^2%2B4%29%28z^2%2B2z%2B2%29%29%29++from+-+infinity+to+infinity



I cannot spot the error.



I get these residues:



Res(1+i)=1+3i20



Res(2i)=2i40




which I add up and multiply by 2πi.



What am I doing wrong?


Answer



Res(2i)=x(x+2i)(x2+2x+2)|x=2i=2i4i(4i2)=14(1+2i)=12i20


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