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Asaf Shachar
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Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$ with isolated, whose zeros inside some open neighbourhood of the closed unit disk $\mathbb{D}^2$ are isolated.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{1,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{1,2}$ and the $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{1,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$ with isolated zeros.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{1,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{1,2}$ and the $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{1,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$, whose zeros inside some open neighbourhood of the closed unit disk $\mathbb{D}^2$ are isolated.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{1,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{1,2}$ and the $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{1,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

I have changed the required convergence rate
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Asaf Shachar
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Can we approximate a vector field on the plane with non-vanishing vector fields in $W^{21,2}$?

Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$ with isolated zeros.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{2,2}$$V_n \in C^\infty \cap W^{1,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{2,2}$$W^{1,2}$ and the $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{2,2}$$W^{1,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

I am interested to know whether stronger convergence is possible.

This question arose in the context of this answer, where the existence of such an approximation is assumed.

Can we approximate a vector field on the plane with non-vanishing vector fields in $W^{2,2}$?

Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$ with isolated zeros.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{2,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{2,2}$ and $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{2,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

I am interested to know whether stronger convergence is possible.

This question arose in the context of this answer, where the existence of such an approximation is assumed.

Can we approximate a vector field on the plane with non-vanishing vector fields in $W^{1,2}$?

Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$ with isolated zeros.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{1,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{1,2}$ and the $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{1,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

Source Link
Asaf Shachar
  • 6.7k
  • 2
  • 20
  • 70

Can we approximate a vector field on the plane with non-vanishing vector fields in $W^{2,2}$?

Let $V$ be a compactly-supported vector field on $\mathbb{R}^2$ with isolated zeros.

Does there exist a sequence of vector fields $V_n \in C^\infty \cap W^{2,2}$ on $\mathbb{R}^2$, such that $V_n \to V$ in $W^{2,2}$ and $V_n$ do not vanish on $\mathbb{D}^2$?

If we replace the $W^{2,2}$ convergence with $L^2$ convergence, than the answer is positive. The idea is to push the zeroes out of the disk by composing $V$ with a diffeomorphism which affects a region of very small measure.

I am interested to know whether stronger convergence is possible.

This question arose in the context of this answer, where the existence of such an approximation is assumed.