cellLimited<cubic> limiter is not differentiable

The cubic polynomial in cubicGradientLimiter.H is

((ar + b)r + 1)r \\
a = \frac{2}{r_t^2} - \frac{2}{r_t^3} \\
b = -\frac32 a r_t

but surely it should be

((ar + b)r + 1)r \\
a = \frac{1}{r_t^2} - \frac{2}{r_t^3} \\
b = -\frac32 a r_t - \frac{1}{2r_t}

to satisfy the constraints from Michalak and Ollivier-Gooch (2008, p.5), specifically that the derivative at the transition point r_t should be zero so it joins up smoothly with 1. This problem is illustrated in the graph below:

Linking this cfd-online thread here just in case someone replies to it after I create this issue.