A Seasonal Adjustment Principle and a Seasonal Adjustment Method Derived From this Principle

The decomposition of a given time series into trend, seasonal
component, and irregular component is formulated as a minimization
problem. The trend is chosen such that it is as smooth as possible;
the seasonal component is chosen such that it exhibits a seasonal
pattern as stable as possible; and the trend and seasonal
components are jointly chosen such that the given time series is
explained as well as possible by these two components; that is, the
irregular component is minimized