>> e_1 tensor e_2

e_1 and e_2 are two 2x1 vectors, [1,0]^T and [0,1]^T, respectively.

e_1 tensor e_2 = a 2x2 matrix, where the first row is (1 * [0, 1]) and the second row is (0 * [0,1]).

e_1 tensor e_2 maybe should be written as e_1 tensor e_2^T ?

It is good to understand the definition of e_1 tensor e_2.

e_i ([1,0]) tensor e_j ([0,1]) 到底變成是什麼東西？