The value of bitcoin, encrypted software that functions as a virtual currency for online transactions, ranged between $55 and $266 this week in trading on the Mt. Gox exchange. The exchange also experienced a distributed-denial-of-service attack and suspended trading Thursday. "It is rare that we get to see a bubble-like phenomenon trade tick for tick in real time," UBS broker Art Cashin reportedly wrote in a note to clients about bitcoin.

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