Mean value of n_i*n_j for a random unit vector in 3D space

[begyu85]

vector mean value - help!

Homework Statement

Let n = (n_1, n_2, n_3) be a random unit vector in Descartes coordinates in the 3-dimensional space.

What is the mean value (or expectation value) of n_i*n_j, where i,j = 1,2,3.

Or shortly: < n_i*n_j > = ?

 

[physixguru]

Welcome To Pf.

Pls Show Some Work.

 

[begyu85]

i think, this mean value is a tensor of 3x3

 

[HallsofIvy]

Why do you think that? You still haven't shown any work.