Given an m -by-n matrix M with entries Mij , we get an n -by-m matrix MT whose ij th entry is Mji , i.e. (MT)ij=Mji
10. Dot product, 2
10. Dot product, 2
Transposition
You may have noticed that the definition of the dot product looks a lot like matrix multiplication. In fact, it is a special case of matrix multiplication: v1w1+⋯+vnwn=(v1⋯vn)(w1⋮wn).
Here, we took the column vector v=(v1⋮vn) and turned it on its side to get a row vector which we call the transpose of v , written: vT=(v1⋯vn)
More generally, you can transpose a matrix:
(1234)T=(1324).
(123456)T=(142536) .
So the rows of M become the columns of MT .
With all this in place, we observe that the dot product v⋅w is vTw .
(AB)T=BTAT .
Writing out the ij th entry of (AB)T using index notation, we get: (AB)Tij=(AB)ji=∑kAjkBki
The order of these factors doesn't matter: Ajk and Bki are just numbers (entries of A and B ), so they commute. This is one reason index notation is so convenient: it converts expressions involving noncommuting objects like matrices into expressions involving commuting quantities (numbers).