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[Calculus] Vector Laplacian Operator

2021-09-19 19:42 作者:AoiSTZ23  | 我要投稿

By: Tao Steven Zheng (鄭濤)

【Problem】

Let %20%5Cboldsymbol%7BA%7D%20%3D%20P(x%2Cy%2Cz)%20%5Cboldsymbol%7Bi%7D%20%2B%20Q(x%2Cy%2Cz)%20%5Cboldsymbol%7Bj%7D%20%2B%20R(x%2Cy%2Cz)%20%5Cboldsymbol%7Bk%7D%20 be a vector field with components that are continuous second partial derivatives in three dimensional space. The vector Laplacian operator is defined as %7B%5Cnabla%7D%5E%7B2%7D%20%5Cboldsymbol%7BA%7D%20%3D%20%5Cnabla(%5Cnabla%20%5Ccdot%20%5Cboldsymbol%7BA%7D)%20-%20%5Cnabla%20%5Ctimes%20(%5Cnabla%20%5Ctimes%20%5Cboldsymbol%7BA%7D)%20. Express the vector Laplacian operator in terms of P%2C%20Q%2C%20R.

【Solution】

Calculate %20%5Cnabla(%5Cnabla%20%5Ccdot%20%5Cboldsymbol%7BA%7D):

%5Cnabla%20%5Ccdot%20%5Cboldsymbol%7BA%7D%20%3D%20P_x%20%2B%20Q_y%20%2B%20R_z

%5Cnabla(%5Cnabla%20%5Ccdot%20%5Cboldsymbol%7BA%7D)%20%3D%20%5Cbegin%7Bpmatrix%7D%20P_%7Bxx%7D%20%2B%20Q_%7Byx%7D%20%2B%20R_%7Bzx%7D%20%5C%5C%20P_%7Bxy%7D%20%2B%20Q_%7Byy%7D%20%2B%20R_%7Bzy%7D%20%5C%5C%20P_%7Bxz%7D%20%2B%20Q_%7Byz%7D%20%2B%20R_%7Bzz%7D%20%5Cend%7Bpmatrix%7D


Calculate %5Cnabla%20%5Ctimes%20(%5Cnabla%20%5Ctimes%20%5Cboldsymbol%7BA%7D)%20:

%5Cnabla%20%5Ctimes%20%5Cboldsymbol%7BA%7D%20%3D%20%5Cbegin%7Bpmatrix%7D%20R_y%20-%20Q_z%20%5C%5C%20P_z%20-%20R_x%20%5C%5C%20Q_x%20-%20P_y%20%5Cend%7Bpmatrix%7D

%5Cnabla%20%5Ctimes%20(%5Cnabla%20%5Ctimes%20%5Cboldsymbol%7BA%7D)%20%3D%20%5Cbegin%7Bpmatrix%7D%20Q_%7Bxy%7D%20-P_%7Byy%7D%20-%20P_%7Bzz%7D%20%2B%20R_%7Bxz%7D%20%5C%5C%20-Q_%7Bxx%7D%20%2B%20P_%7Byx%7D%20%2B%20R_%7Byz%7D%20-%20Q_%7Bzz%7D%20%5C%5C%20P_%7Bzx%7D%20-%20R_%7Bxx%7D%20-%20R_%7Byy%7D%20%2B%20Q_%7Bzy%7D%20%5Cend%7Bpmatrix%7D%20

Consequently,?

%5Cnabla(%5Cnabla%20%5Ccdot%20%5Cboldsymbol%7BA%7D)%20-%20%5Cnabla%20%5Ctimes%20(%5Cnabla%20%5Ctimes%20%5Cboldsymbol%7BA%7D)%20%3D%20%5Cbegin%7Bpmatrix%7D%20P_%7Bxx%7D%20%2B%20P_%7Byy%7D%20%2B%20P_%7Bzz%7D%20%2B%20(R_%7Bzx%7D%20-%20R_%7Bxz%7D)%20%2B%20(Q_%7Byx%7D%20-%20Q_%7Bxy%7D)%20%5C%5C%20Q_%7Bxx%7D%20%2B%20Q_%7Byy%7D%20%2B%20Q_%7Bzz%7D%20%2B%20(P_%7Bxy%7D%20-%20P_%7Byx%7D)%20%2B%20(R_%7Bzy%7D%20-%20R_%7Byz%7D)%20%5C%5C%20R_%7Bxx%7D%20%2B%20R_%7Byy%7D%20%2B%20R_%7Bzz%7D%20%2B%20(P_%7Bxz%7D%20-%20P_%7Bzx%7D)%20%2B%20(Q_%7Byz%7D%20-%20Q_%7Bzy%7D)%20%5Cend%7Bpmatrix%7D

By Clairaut's theorem of mixed partials, each bracket in the above vector equal zero; therefore,

%20%5Cnabla(%5Cnabla%20%5Ccdot%20%5Cboldsymbol%7BA%7D)%20-%20%5Cnabla%20%5Ctimes%20(%5Cnabla%20%5Ctimes%20%5Cboldsymbol%7BA%7D)%20%3D%0A%5Cbegin%7Bpmatrix%7D%20P_%7Bxx%7D%20%2B%20P_%7Byy%7D%20%2B%20P_%7Bzz%7D%20%5C%5C%20Q_%7Bxx%7D%20%2B%20Q_%7Byy%7D%20%2B%20Q_%7Bzz%7D%20%5C%5C%20R_%7Bxx%7D%20%2B%20R_%7Byy%7D%20%2B%20R_%7Bzz%7D%20%5Cend%7Bpmatrix%7D

This expression can be compactly expressed as

%7B%5Cnabla%7D%5E%7B2%7D%20%5Cboldsymbol%7BA%7D%20%3D%20%5Cbegin%7Bpmatrix%7D%20%7B%5Cnabla%7D%5E%7B2%7D%20P%20%5C%5C%20%7B%5Cnabla%7D%5E%7B2%7D%20Q%20%5C%5C%20%7B%5Cnabla%7D%5E%7B2%7D%20R%20%5Cend%7Bpmatrix%7D

where the left-hand side is the vector Laplacian of %5Cboldsymbol%7BA%7D, and the right-hand side is a vector of the scalar Laplacian of the components of %5Cboldsymbol%7BA%7D%20.

[Calculus] Vector Laplacian Operator的評論 (共 條)

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