Matrix multiplications

Questions
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1

HI everyone,

This might be really trivial for most people but i cant seem to solve it.
I have lets say two matrices

Ix = (1/2)*{{0, 1}, {1, 0}} // MatrixForm

Iy = (1/2)*{{0, -i}, {i, 0}} // MatrixForm

so i do

MatrixForm[Ix.Iy]

I do not actually get a product of Matrics but just a form which gives me Ix.Iy in matrix form?

I can’t seem to figure out what i am doing wrong.

Thanks for the help?

2

The postfix operator (//) has a higher precedence than Set (=), so Ix and Iy are not just the matrix, but the matrix with the formatting function MatrixForm:

In[7]:= FullForm[Ix]
Out[7]//FullForm= MatrixForm[List[List[0,Rational[1,2]],List[Rational[1,2],0]]]

You can use parentheses in this case to avoid this problem:

(Ix = (1/2)*{{0, 1}, {1, 0}}) // MatrixForm
(Iy = (1/2)*{{0, -i}, {i, 0}}) // MatrixForm
MatrixForm[Ix.Iy]
3

Dear Arnoud

Thanks for your kind reply. That clears up the idea.