Here, prof. Andrew shows the shape of the curve for the cross-validation error for different degrees of a polynomial model, and also for the regularization parameter.

The curve here looks like a unimodal function for both curves.

Ternary search is an algorithm that is used to find the minimum (or the maximum) of a unimodal function.

Aside from the fact that training models on different parameters is not cheap, and that this idea might take so long, on an abstract level, is it correct?