$D$-optimal designs of experiments for trigonometric regression on interval with heteroscedastic observations

In article the problem of construction continuous (number of observations is not fixed) $D$-optimal designs of experiments for trigonometric regression in a case when variance of errors of observations depend on a point in which is made is investigated. Class of functions which describe change variance of heteroscedastic observations is defined for which it is possible construct continuous $D$-optimal designs of experiments. For trigonometric regression with three factors it is constructed continuous $D$-optimal designs of experiments with different types heteroscedastic observations. For each of these types the own class of functions describing change variance of observations is defined.