  The complex networks approach has been gaining popularity in analysing
investor behaviour and stock markets, but within this approach, initial public
offerings (IPO) have barely been explored. We fill this gap in the literature
by analysing investor clusters in the first two years after the IPO filing in
the Helsinki Stock Exchange by using a statistically validated network method
to infer investor links based on the co-occurrences of investors' trade timing
for 69 IPO stocks. Our findings show that a rather large part of statistically
similar network structures form in different securities and persist in time for
mature and IPO companies. We also find evidence of institutional herding.
