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Uber has done something that few other big technology companies can claim: open below its initial public offering price.

Shares of the ride-sharing company began trading Friday at $42 a share, down 6.7 percent, and finished the day at $41.57, well below the $45 at which it sold shares to investors late Thursday.

How a stock trades in its debut is often used to gauge the success of the offering and investor sentiment about a company. If a stock falls below its I.P.O. price on the first day or even in the weeks that follow, the offering is considered a flop — a red flag that investors have concerns about a company’s outlook.