Therefore a rather detailed discussion of this topic
is presented in every course on quantum mechanics.The fact
that the different components of the angular momentum do
not commute allows us, in addition,to discuss the basic facts
about measurements in quantum systems.It is surprising that
most textbooks only show that the expectation values of jˆ x
and jˆ y vanish in eigenstates u j,mz& of jˆ2 and jˆ z and present
the result for ^ j,mzujˆ
x
2u j,mz&, but give no detailed discussion
of the measurement of jˆ x for a given u j,mz&.