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

`common_xregs`

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. |

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"). |

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. |