The YNOT formula

This formula gives an approximation for the perimeter of an ellipse.

The perimeter of an ellipse with half-axes a and b is obtained as:

                                   4 ( a^y + b^y)^1/y

with y = ln(2)/ln(pi/2) ~ 1.534928536...(y is called the YNOT-constant)

Rewritten as a function of a and the excentricity,e, this formula becomes:

                                   4a ( 1 + (1-e²)^y/2)^1/y

This approximation formula, first proposed by Roger Maertens, makes an error of at most 0.4%.