Vice President Joe Biden is opening a three-day summit at the White House aimed at countering the spread of violent extremism across the globe. Biden will take part in a round-table discussion Tuesday with local leaders from Boston, Los Angeles and Minneapolis. The three cities have programs to counter extremism that the White House wants to promote as examples for the rest of the country. The conference continues Wednesday with a speech by President Barack Obama about how cities are tackling the problem. Obama will also address representatives from about 60 countries Thursday at the State Department.