What is the largest prime integer known to man that is NOT one less than a power of two?

arithmetic/largest.non.mersenne.prime New

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Problem

What is the largest prime integer known to man that is NOT one less
than a power of two?

-- Wei-Hwa Huang <whuang@ugcs.caltech.edu>, April 1997

Show solution

For prime number information, a good place to start is Chris
Caldwell's "Prime Page" at <http://www.utm.edu/research/primes/>.

The largest known non-Mersenne prime is 391581*2^216193-1, discovered
in 1989 by a group working at Amdahl.  At 65087 digits, it exceeded the
largest known Mersenne prime by just 37 digits, and was the largest
known prime for two and a half years.

-- Fred W. Helenius <fredh@ix.netcom.com>, April 1997

[ Editor: this was the record as of 1997. ]