This tutorial talks about linear regression with harmonic seasonality.

### 1 Underlying mathematics

In regression modeling with seasonality, we can use one parameter for each season. For instance, 12 parameters for 12 months in one year. However, seasonal effects often vary smoothly over the seasons, so that it may be more parameter-efficient to use a smooth function instead of separate indices. *Sine* and *cosine* functions can be used to build smooth variationinto a seasonal model. Read more R in Time Series: Linear Regression With Harmonic Seasonality