From the standpoint of the system used by us, the task also arises to derive the momentum density from relation (18). It follows from (36)
Thus the right-hand side of (18) becomes
(38)
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We express, on the basis of (37),
and
by the vectors arising in the main equations, and we find
(38a)
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(38b)
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By inserting (38a,b) into (38), we obtain
(38c)
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However, now it follows from (36)
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thus the second and third row of the right-hand side of (28c) assume the values
If it indeed holds, as required by (18a)
(39)
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then the second and third row are providing together:
Therefore it eventually follows from (18)
(39a)
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The comparison with (20) gives the important relation
(40)
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