These special functions provide interfaces to more complicated functions within the model formulae interface.

common_xregs

## Specials

### trend

The trend special includes common linear trend regressors in the model. It also supports piecewise linear trend via the knots argument.

trend(knots = NULL, origin = NULL)

 knots A vector of times (same class as the data's time index) identifying the position of knots for a piecewise linear trend. origin An optional time value to act as the starting time for the trend.

### season

The season special includes seasonal dummy variables in the model.

season(period = NULL)

 period The periodic nature of the seasonality. This can be either a number indicating the number of observations in each seasonal period, or text to indicate the duration of the seasonal window (for example, annual seasonality would be "1 year").

### fourier

The fourier special includes seasonal fourier terms in the model. The maximum order of the fourier terms must be specified using K.

fourier(period = NULL, K, origin = NULL)

 period The periodic nature of the seasonality. This can be either a number indicating the number of observations in each seasonal period, or text to indicate the duration of the seasonal window (for example, annual seasonality would be "1 year"). K The maximum order of the fourier terms. origin An optional time value to act as the starting time for the fourier series.