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In general, the ket is not a constant multiple of . However, there are some special kets known as the eigenkets of operator . These are denoted
and have the property
where , , are numbers called eigenvalues. Clearly, applying to one of its eigenkets yields the same eigenket multiplied by the associated eigenvalue.
Consider the eigenkets and eigenvalues of a Hermitian operator . These are denoted
where is the eigenket associated with the eigenvalue . Three important results are readily deduced:
(i) The eigenvalues are all real numbers, and the eigenkets corresponding to different eigenvalues are orthogonal. Since is Hermitian, the dual equation to Eq. (44) (for the eigenvalue ) reads
If we left-multiply Eq. (44) by , right-multiply the above equation by , and take the difference, we obtain
Suppose that the eigenvalues and are the same. It follows from the above that
where we have used the fact that is not the null ket. This proves that the eigenvalues are real numbers. Suppose that the eigenvalues and are different. It follows that
which demonstrates that eigenkets corresponding to different eigenvalues are orthogonal.
(ii) The eigenvalues associated with eigenkets are the same as the eigenvalues associated with eigenbras. An eigenbra of corresponding to an eigenvalue is defined
(iii) The dual of any eigenket is an eigenbra belonging to the same eigenvalue, and conversely.