In lecture 4 (i didn't watch it because I had no idea what transposing is, which was not mentioned in Lecture 3) do we learn LU decomposition? I'm staring at the textbook and I'm sort of confused.

Question

epoweritheta · Answer

yes , Prof Strang discusses LU decomposition in lecture 4 , 
transpose of a matrix A is a matrix with columns the rows of A ,ie row 1 col A' is col 1 of A

anonymous · Answer

In the textbook, section 2.7 (page 107) explains transposes.  The easy trick is that the numbers are all the same -- they just change position.