ABC2019 wrote:

But I do not see the relation with the equations of the 3rd degree.

it is impressive to present oneself as the only scientist of this forum and not to recognize a 3rd degree equation !!

1 + f + f ^ 2 = 1 / (1-f) is not equivalent to a: (1-f) x (1 + f + f ^ 2) - 1 = 0 !! (either a + bx + cx ^ 2 + dx ^ 3 = 0)?

I gave the solutions without knowing anything about the climate!