Hi, i separated parity question from my question posted just below Parity operator reveal parity of function such that [itex] Pf = \pm f [/itex] But In fact, in order to get parity of function f, we should already know parity of function In that way, Why should we know Parity operator ? we already know parity of function! Second, since partiy operator commute with Hamilitonian, expectation value of Parity is conserved in time. What does it means? Why is it so important even author assign 5 page to make it clear.