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 14 IPO stocks. Our findings show that a rather large part of statistically similar network structures is persistent and general: they form in different securities' and persist in time for mature and IPO companies. We also find evidence of institutional herding that hints at the existence of an investor information network.
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