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The numbers 4, 8, 15, 16, 23 and 42 appear throughout the series, both in sequence and individually. They were broadcast from the Island's radio transmitter, and it was this message that drew Rousseau's expedition there. Although she later changes the message after the deaths of the rest of her team, the digits had also been heard by other people, eventually making their way to Hurley, who used them to win a lottery. After those around him suffer a series of misfortunes, he begins to believe the numbers are cursed. His search for their origin leads him to Australia and, through the crash, to the island, where he ultimately discovers them engraved on the hatch. They also appear inside the bunker, on medicine bottles, and constitute a code that must be entered into the Swan station's computer. The sum of these numbers, 108, has also become significant in connection to the DHARMA Initiative. It appears on a mural inside the Initiative's Station Three, and the full sequence of numbers must be entered into its computer every 108 minutes.

Source: Wikipedia.org