k is the modulus of the vector k. Please help me to integrate the above tensor expression in the infinite domain of the vector k. I have tried to let u in the direction of kz and then transform the current integral into a spherical coordinate with the following relation:
And I find Q is a symmetric tensor, and Qxy=0,Qxz=0,Qyz=0. However,Qxx=Qyy≠0,Qzz≠0,and Qxx≠Qzz. I guess Q has relevance to the tensor uu, the assumption that u is in the direction of kz is just a simple case.How to conduct this integral in the general case is my question.Thank you very much.