  We investigate the problem of counting co-authorhip in order to quantify the
impact and relevance of scientific research output through normalized
\textit{h-index} and \textit{g-index}. We use the papers whose authors belong
to a subset of full professors of the Italian Settore Scientifico Disciplinare
(SSD) FIS01 - Experimental Physics. In this SSD two populations, characterized
by the number of co-authors of each paper, are roughly present. The total
number of citations for each individuals, as well as their h-index and g-index,
strongly depends on the average number of co-authors. We show that, in order to
remove the dependence of the various indices on the two populations, the best
way to define a fractional counting of autorship is to divide the number of
citations received by each paper by the square root of the number of
co-authors. This allows us to obtain some information which can be used for a
better understanding of the scientific knowledge made through the process of
writing and publishing papers.
