FK理論
∂ θ ∂ τ = 1 δ 1 η j ∂ ∂ η ( η j ∂ θ ∂ η ) + e θ / ( 1 + θ / β ) {\displaystyle {\frac {\partial \theta }{\partial \tau }}={\frac {1}{\delta }}{\frac {1}{\eta ^{j}}}{\frac {\partial }{\partial \eta }}\left(\eta ^{j}{\frac {\partial \theta }{\partial \eta }}\right)+e^{\theta /(1+\theta /\beta )}}
Since β ≫ 1 {\displaystyle \beta \gg 1} ,指数項を線形化すると e θ / ( 1 + θ / β ) ≈ e θ {\displaystyle e^{\theta /(1+\theta /\beta )}\approx e^{\theta }} となる。
∂ θ ∂ τ = 1 δ 1 η j ∂ ∂ η ( η j ∂ θ ∂ η ) + e θ {\displaystyle {\frac {\partial \theta }{\partial \tau }}={\frac {1}{\delta }}{\frac {1}{\eta ^{j}}}{\frac {\partial }{\partial \eta }}\left(\eta ^{j}{\frac {\partial \theta }{\partial \eta }}\right)+e^{\theta }}