What is the equation for flat shading? Is there an equation for flat shading?

To my understanding, it is this:

$$I = I_a \times K_a + F_{att} \times  I_l \times  (K_d \times  (N \times  L))$$

Final intensity = ambient reflection x ambient reflection coefficient + (attenuation x incident light intensity x (diffusive reflection coefficient x (surface normal x Light intensity))

However, I have only found one source for this equation (my lecture slides).

Are there any other sources to confirm this is the equation for flat shading? I can't find one.
I don't think having a equation for flat shading is meaningful as such. Mainly because the formula for flat shading is same as the formula for the model the flat shading is emulating. The difference is that flat shading is operating per primitive rather than per point or per vertex.

So there can in fact exist multiple formulas that work differently but are still all flat shading.