2011-06-04, 08:45 AM
2147483647 Wrote:SpoilerInterestingly, WolframAlpha says that the integral is θ+i*log(1+3exp(iθ
). Then we run into the problem of taking the log the form that I was having trouble with earlier, namely the one I bolded and inferred:
log(3exp(4iπ+1) - log(3exp(0i)+1) = 4πi
but e^(4 pi i) = e^(2 pi i) = e^(0 pi i) = 1, so log(3e^(4 i pi) + 1) = log(3 + 1) = log(3 e^(0 pi i) + 1), and that leaves us with zero?
2147483647 Wrote:This is precisely what I did, without shifting the "index" (aka I parametrized every line starting from (1,1) from 1 to -1 and then from -1 to 1 when I reached (-1,-1). I do see the problem now, however. In the last step, simplification had to be grouped:
∮1/z dz = ∫ 1/(t+i) dt [1,-1] + ∫ i/(-1+it) dt [1,-1] + ∫ 1/(t-i) dt [-1,1] + ∫ i/(1+it) dt [-1,1]
= ln(t+i) [1,-1] + ln(-1+it) [1,-1] + ln(t-i) [-1,1] + ln(1+it) [-1,1]
= {ln(1+i) - ln(-1+i)} + {ln(-1+i) - ln(-1-i)}+ {ln(-1-i) - ln(1-i)} + {ln(1-i) - ln(1+i)}
= ln[(1+1)/(-1+i)] + ln[(-1+i)/(-1-i)] + ln[(-1-i)/(1-i)] + ln[(1-i)/(1+i)]
And you claim that each one of these grouped logarithms is equivalent to πi/2.
No, I claim that specific result to be -2 pi i as you went clockwise instead of counterclockwise.
Also, Wolfram Alpha is partly correct, but look up complex logarithms! Some of them have infinitely many solutions.
Noah


). Then we run into the problem of taking the log the form that I was having trouble with earlier, namely the one I bolded and inferred: