Hello,

the problem is in the default optimization setting of the definition of M. By default, optimization is set to "symbolic" in definitions (assignments with :=) and to "numeric" in evaluations by "=". As such, this should not create any problems if the symbolic engine was reliable. Unfortunately it is not. In particular, the engine is unaware of the different meaning of the multiplication dots depending on operator type. You can see that by removing the defintions of the vectors and hovering with the mouse over the definition of M. As you see, the braces are gone.
If you switch to numeric in the context menu, you get the correct result.

This is not yet the whole story. If you set a defintion to symbolic and evaluate it later in the sheet, the value depends on the variables as defined at the place, where the evaluation is made. That means, if you change the vectors a, b or c later, M has different ddisplayed values beyond that changes.

Now, if your definition is set to numeric, then two things may happen:
a ) all variables in the definition have a value, then M is evaluated immediately and the result is stored as value of M.
Later modifications of the variables do not have any effect.
b ) at least one variable is undefined. Then the definition is stored as given (without spoiling it by wrong simplifications) and you get the expected behaviour.

Again, all this can be seen in the dynamic assistant or in the definition preview (mouse hovering)

Edited by user 09 March 2013 14:57:59(UTC)  | Reason: Not specified

If there was a dot product operator just for vectors, we could write
a * (b dot c)
and symbolic engine would not simplify it to
a * b * c
which means
(a dot b ) * c

Another workaround therefore is to define a dot product on our own:

dot(u,v):sum(el(u,k)*el(v,k),k,1,rows(u))

And we get what we were looking for:

M3:mat(dot(a,*c,c*dot(a,,2,1)

M3=mat(mat(-30,30,30,3,1),mat(-30,30,30,3,1),2,1)


PS: Simpler version dot(u,v):u*v didn't work - it is also substituted and we get a wrong result.