- last one to model varying phenotypic optima. Be careful that large
- values of \code{alpha} may give unrealistic output.}
+ last one to model varying phenotypic optima.
+
+ By default the following formula is used:
+
+ \deqn{x_{t''} = x_{t'} - \alpha l (x_{t'} - \theta) + \sigma
+ l \epsilon}{x(t'') = x(t') - alpha l (x(t') - theta) + sigma
+ l epsilon}
+
+ where \eqn{l (= t'' - t')} is the branch length, and \eqn{\epsilon
+ \sim N(0, 1)}{\epsilon ~ N(0, 1)}. If \eqn{\alpha > 1}{alpha > 1},
+ this may lead to chaotic oscillations. Thus an alternative
+ parameterisation is used if \code{linear = FALSE}:
+
+ \deqn{x_{t''} = x_{t'} - (1 - exp(-\alpha l)) * (x_{t'} - \theta) +
+ \sigma l \epsilon}{x(t'') = x(t') - (1 - exp(-alpha l)) * (x(t') -
+ theta) + sigma l epsilon}}