- #1
mahler1
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Homework Statement .
Let
##D =\left\{\left(x,y\right)\ \in\ {\mathbb R}^{2}: x^{2} + y^{2} \leq 1\right\}## and let ##{\rm f}:D \to {\mathbb R}## defined as
##{\rm f}\left(x,y\right)
= \left(1 - x^{2} -y^{2}\right)\exp\left(x^{4}y^{10}\right)##.
Consider the surface ##S## given by the graph of ##{\rm f}## restricted to ##D##, oriented with the exterior normal vector. Let ##G## be the vector field
##G:{\mathbb R}^{3} \to {\mathbb R}^{3}## given by
##G(x,y,z) = \left(\,-y,\, x,\, x^{2} + y^{2}\,\right)\,,\qquad\mbox{Calculate}\ \int_{S} G\cdot dS##
The attempt at a solution.
I am having a hard time trying to visualize the surface ##S##.
One possibility is to use Gauss theorem, which says that if ##W## is an elementary symmetric region in space where ##\partial W## is a closed oriented surface and ##F## is a function of class ##C^1##, then
##\iiint_w (divF).dV=\iint_{\partial W} F.dS##.
I've calculated the divergence of ##G## and it gives ##0##. If I could find a region ##W## bounded by ##S##, then
##0=\iiint_w (divG).dV=\iint_{\partial W} G.dS=\int_S G.dS##
Another doubt that I have is: how do I know ##S## is a closed surface?
Let
##D =\left\{\left(x,y\right)\ \in\ {\mathbb R}^{2}: x^{2} + y^{2} \leq 1\right\}## and let ##{\rm f}:D \to {\mathbb R}## defined as
##{\rm f}\left(x,y\right)
= \left(1 - x^{2} -y^{2}\right)\exp\left(x^{4}y^{10}\right)##.
Consider the surface ##S## given by the graph of ##{\rm f}## restricted to ##D##, oriented with the exterior normal vector. Let ##G## be the vector field
##G:{\mathbb R}^{3} \to {\mathbb R}^{3}## given by
##G(x,y,z) = \left(\,-y,\, x,\, x^{2} + y^{2}\,\right)\,,\qquad\mbox{Calculate}\ \int_{S} G\cdot dS##
The attempt at a solution.
I am having a hard time trying to visualize the surface ##S##.
One possibility is to use Gauss theorem, which says that if ##W## is an elementary symmetric region in space where ##\partial W## is a closed oriented surface and ##F## is a function of class ##C^1##, then
##\iiint_w (divF).dV=\iint_{\partial W} F.dS##.
I've calculated the divergence of ##G## and it gives ##0##. If I could find a region ##W## bounded by ##S##, then
##0=\iiint_w (divG).dV=\iint_{\partial W} G.dS=\int_S G.dS##
Another doubt that I have is: how do I know ##S## is a closed surface?