  Recent advances in the understanding of time series permit to clarify
seasonalities and cycles, which might be rather obscure in today's literature.
A theorem due to P. Cartier and Y. Perrin, which was published only recently,
in 1995, and several time scales yield, perhaps for the first time, a clear-cut
definition of seasonalities and cycles. Their detection and their extraction,
moreover, become easy to implement. Several computer experiments with concrete
data from various fields are presented and discussed. The conclusion suggests
the application of this approach to the debatable Kondriatev waves.
