Inductance as a function of position

[Jimbo]

Hello
I am doing some research on non-linear systems, and have come across a equation regarding inductance and I am unsure how it was derived:[p]
The inductance of the electromagnet depends on the position of the magnetic material, and can be modelled as,
L(y) = L1 + L0 / (1 + (y / a))
where y is the position of the material from a reference point, and L1, L0, and a are positive constants
Is this equation an adaptation of a more fundamental equation?
I get the gist that, as the material gets further away the inductance decreases and vice versa, but am just unsure about how the equation was formed?
Thanks for any guidance
Jimbo

[Jimbo]

Sorry, looking at it again I think I understand:

As the position decreases the denominator will approach 1, making L0 (I assume the inductance when very close) to make the biggest impact on the L(y) value. If y was very large, the denominator would be large, limiting L0s affect on the equation.

Sorry if my post was a bit of a waste of space :(

Jimbo