KRVS

Shankar Vedantam

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Magic.

That's what it feels like when you bump into your childhood friend on the first day of college ... or meet someone at a party in Paris, only to discover she lives in your dad's childhood home in Poughkeepsie, N.Y. But mathematician Joseph Mazur says these coincidences are not as extraordinary as we might think.

"People think that their address book is essentially the people they know, and it turns out any address book is about one percent of the people they know in some way," Mazur explains.

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