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President Barack Obama greets Republican presidential nominee Mitt Romney at the start of the second presidential debate at Hofstra University, Tuesday, Oct. 16, 2012, in Hempstead, N.Y. (AP Photo/David Goldman)

HEMPSTEAD, N.Y. (AP) — The second presidential debate is under way.

President Barack Obama and Mitt Romney are squaring off for the second time of three planned debates.

They're appearing in Hempstead, N.Y., with undecided voters asking questions.

It's a nationally televised town hall-style encounter exactly three weeks before Election Day.