The outputs are created according to the formula $$y = \mbox{atan}((x2 x3 - (1/(x2 x4)))/x1) + e$$
where e is N(0,sd).
mlbench.friedman3(n, sd=0.1)
Friedman, Jerome H. (1991) Multivariate adaptive regression splines. The Annals of Statistics 19 (1), pages 1-67.