IRP=4 Grant's curve

IRP = 4 Grant's curves (Grant, 1977)

${k_{rl}}\quad = \quad {\hat S^4}$

${k_{rg}}\quad = \quad {\left( {1\; - \;\hat S} \right)^2}\;\left( {1\; - \;{{\hat S}^2}} \right)$

where

$\hat S = \frac{{\left( {{S_l}\; - \;{S_{lr}}} \right)}}{{\left( {{S_l}\; - \;{S_{lr}}} \right)}}$

with Slr = RP(1); Sgr = RP(2)

Restrictions: RP(1) + RP(2) < 1