Effect of Air gap in Flyback transformer on primary inductance

Could anyone please explain how the air gap in the flyback transformer really effects the primary winding inductance. I am basically looking for the mathematical relation...

Thanks in advance,
Embedded_Geek

Calculating the reluctance of the magnetic circuit is a visual method to find the inductance.

http://en.wikipedia.org/wiki/Magnetic_circuit

I guess you know about AL factors of magnetic cores. According to the reluctance model, you can assume

1/ALgapped = 1/ALungapped + k*sg (sg is the gap width, k a constant)

Due to fringing fields, the calculation isn't exact for very large air gaps.