一个刚性常微分方程。
火焰（R0）指定初始半径R0。默认的R0 = 02。
一个火球会膨胀，直到它的半径足够大，所有的氧气都可以通过地表燃烧。半径的方程^ RDOT = R 2 R ^ 3。(A stiff ordinary differential equation.
FLAME(r0) specifies the initial radius is r0.　Default r0 = .02.
A ball of fire grows until its radius is just large enough that all of the oxygen available through the surface is consumed by combustion in the interior.　The equation for the radius is rdot = r^2 - r^3.)