JPMorgan Chase and Citigroup opened the latest round of major corporate earnings with surprisingly good results and notched big gains in an otherwise weak day for stocks. Their results gave investors more to look forward to in the sector as other banks prepare to present their fourth-quarter reports. Both JPMorgan and Citigroup reported a jump in profit because of solid gains from their trading desks. Those same desks had a tough time a year ago, when the stock market declined sharply during the same period. Several other banks were lifted by the solid results. Bank of America, which reports results on Wednesday, rose 1%. Wells Fargo was the sector’s weak link after giving investors a disappointing earnings report.