  This is a self-published methodological note distributed under the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0/),
which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited. The note contains an original
reasoning of mine and the goal to share thoughts and methodologies, not
results. Therefore before using the contents of these notes, everyone is
invited to verify the accuracy of the assumptions and conclusions.
