5 The Wave Equation and the Third Axiom
47
F 21 =
b∈G 2
a∈G 1
f ba =
b∈G 2
a∈G 1
(−f ab ) = −
a∈G 1
b∈G 2
f ab = −F 12 .
Therefore, we have proven that (53) is derived from (55).