  In order to take multiple co-authorship appropriately into account, a
straightforward modification of the Hirsch index was recently proposed.
Fractionalised counting of the papers yields an appropriate measure which is
called the hm-index. The effect of this procedure is compared in the present
work with other variants of the h-index and found to be superior to the
fractionalised counting of citations and to the normalization of the h-index
with the average number of authors in the h-core. Three fictitious examples for
model cases and one empirical case are analysed.
