  Starting from the traditional definition of auto-correlation function we can
introduce a new quantity, based on the wavelet formalism, called local wavelet
auto-correlation function, which contains more information about the
correlation properties on a given signal on different scales. The application
of this formalism to the temporal variations of solar activity, from the
monthly time series of sunspot numbers, allows to derive some interesting
information about the correlation properties of different periodicities present
in the signal and in particular the correlation characteristics of the main 11
years Schwabe cycle. The scale selectivity, allowed by the local wavelet
auto-correlation function, can give interesting information about the range of
reliable predictions reachable through deterministic predictive models.
