U.S. stocks opened little changed on Monday, retreating slightly from last week's strong gains, as oil prices gave up most of their overnight jump. Meanwhile, shares of Pfizer and Allergan PLC fell about 2% after the drug makers said they'll merge in a so-called inversion deal. The S&P 500 opened 0.1 point or less than 0.1%, lower at 2,089. The Dow Jones Industrial Average fell 21 points, or 0.1%, to 17,803. The Nasdaq Composite began the day up 2 points, or less than 0.1%, at 5,106.
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