Time reversal with neglect of electron spin

The operator in square brackets, which is designated by Ql(t — to), tells us how to calculate -ij)(r() + St) from ib(to) and is therefore called the (infinitesimal) time-evolution operator. Since 4t does not depend on /0, 

Equation (8) expresses the composition property of the time-evolution operator, 

When H is independent of t, //( / to), for a finite time interval, can be obtained by applying the composition property repeatedly to n successive time intervals each of length 

2 Time reversal with neglect of electron spin 

Provided H is real, which will be true at a level of approximation that neglects electron spin, 

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