In a study published Thursday in the journal Science, Dr. Johnson and Miami colleagues 
searched for pro-Islamic State posts each day from mid-2014 until August 2015, mining mentions of beheadings and blood baths in multiple languages on Vkontakte, a Russia-based social media service that is the largest European equivalent to Facebook. Ultimately, they devised an equation that tries to explain the activity of Islamic State sympathizers online and might, they say, eventually help predict attacks that are about to happen.