マクスウェル・ベティの相反作用の定理
( 1 2 P A u A A ) + ( 1 2 P B u B B + P A u A B ) = ( 1 2 P B u B B ) + ( 1 2 P A u A A + P B u B A ) ∴ P A u A B = P B u B A {\displaystyle {\begin{aligned}\left({\frac {1}{2}}P_{A}u_{AA}\right)+\left({\frac {1}{2}}P_{B}u_{BB}+P_{A}u_{AB}\right)&=\left({\frac {1}{2}}P_{B}u_{BB}\right)+\left({\frac {1}{2}}P_{A}u_{AA}+P_{B}u_{BA}\right)\\\therefore P_{A}u_{AB}&=P_{B}u_{BA}\\\end{aligned}}}
が成り立つ。