Maybe nice

Calculus Level 4

True or false :

Let ϕ : [ 0 , 2 π ] R 2 , ϕ ( t ) = ( 2 + cos t , sin t ) \phi: [0, 2\pi ] \rightarrow \mathbb R^2 , \quad \phi(t) = (2+ \cos t , \sin t ) and let γ : [ 0 , 2 π ] R 2 , γ ( t ) = ( cos t , sin t ) \gamma: [0, 2\pi ] \rightarrow \mathbb R^2 , \quad \gamma(t) = ( \cos t , \sin t ) , then the equation below is satisfied.

ϕ y x 2 + y 2 d x + x x 2 + y 2 d y = γ y x 2 + y 2 d x + x x 2 + y 2 d y \int_{\phi} \dfrac{-y}{x^2+ y^2} \, dx + \dfrac x{x^2+y^2} \, dy = \int_{\gamma} \dfrac{-y}{x^2+ y^2} \, dx + \dfrac x{x^2+y^2} \, dy

True False

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2 solutions

Otto Bretscher
Feb 27, 2016

Nice problem! Please do post more problems on vector calculus.

ϕ ω = 0 \int_{\phi}\omega=0 by Stokes' Theorem since d ω = 0 d\omega=0 for the given 1-form ω \omega . Note that this form is defined throughout the disc M M enclosed by the circle M \partial M . (Recall that Stokes' Theorem states that M ω = M d ω \int_{\partial{M}}\omega=\int_{M}d\omega )

Using the given parameterization, we see that γ ω = 0 2 π ( sin 2 t + cos 2 t ) d t = 2 π \int_{\gamma}\omega=\int_{0}^{2\pi}(\sin^2t+\cos^2t)dt=2\pi . Here we cannot use Stokes' Theorem directly since the form is undefined at the origin.

Thus the stated equation is F a l s e \boxed{False} .

Moderator note:

Great! When applying a theorem, we always have to check that the initial conditions hold. It is tempting to assume that they do.

Thank you Sir..Nice solution . I will post for sure.. Thanks

Jose Sacramento - 5 years, 3 months ago

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In the problem, should the second parametriation be γ ( t ) = ( cos t , sin t \gamma(t) = ( \cos t, \sin t ) instead?

Calvin Lin Staff - 5 years, 3 months ago

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Yes. You are right Calvin.. I made a mistake writing the problem.. I hope the members will understand this mistake of myself. I am sorry.. Cheers

Jose Sacramento - 5 years, 3 months ago

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@Jose Sacramento Thanks. I've edited the problem accordingly. Note that you, as the problem creator, can edit your own problems by selecting that option in the menu.

Calvin Lin Staff - 5 years, 3 months ago
James Wilson
Nov 14, 2017

I recognized the integrand as the imaginary part of 1 x + i y \frac{1}{x+iy} . Since this function is analytic inside and on the closed loop ϕ \phi , the value of the integral is zero (by the Cauchy Integral Theorem). It is not hard to show that the other integral is equal to 2 π 2\pi (whether you simplify the integrand to 1 or just apply the Cauchy Integral Formula). In other words, the integrals are not equal.

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