  An important issue in bibliometrics is the weighing of co-authorship in the
production of scientific collaborations, which are becoming the standard
modality of research activity in many disciplines. The problem is especially
relevant in the field of high-energy physics, where collaborations reach 3000
authors, but it can no longer be ignored also in other domains, like medicine
or biology. We present theoretical and numerical arguments in favour of
weighing the individual contributions as $1/N_{\rm aut}^\alpha$ where $N_{\rm
aut}$ is the number of co-authors. When counting citations we suggest the
exponent $\alpha\approx 1$, that corresponds to fractional counting. When
counting the number of papers we suggest $\alpha \approx 1/3 - 1/2$, with the
former (latter) value more appropriate for larger (smaller) collaborations. We
expect and verify that the $h$ index scales as the square root of the average
number of co-authors, and define a fractionalized $h$ index that does not scale
with collaboration size.
