# What is this shortcut to determine primality?

I'm watching this, he says that David Slowinski discovered the biggest prime in 1984: $2^{132,049}$-1 and that it took 1 week on a Cray supercomputer: using some shortcut and that the absence of this shortcut would make the process last the age of the universe. What's this shortcut?

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Numbers of the form $2^n-1$ can be tested by the Lucas-Lehmer method, which is not available for numbers not of that form.
I have edited in a link to the description at Wikipedia, but you could just type $$\rm Lucas\ Lehmer$$ into the internet and take your pick of places to look. –  Gerry Myerson Dec 6 '12 at 2:29