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4. The operatorsF should form an algebra, We do not specify this algebra.
5. We should assume also thatx 0 does not depend onx. If this condition is not satisfied, eq. (10) gets changed, but the decay factor can be simplified. For example, if we assume thatx μ−x μ 0 is parallel topμ, the decay factor depends only on the differencet−t 0.