Mersenne Number whose Index is Mersenne Prime/Mistake

Source Work

 * The Dictionary
 * $8191$
 * $8191$

Mistake

 * It had been conjectured that although most Mersenne number appear to be composite, a Mersenne number whose index was prime would itself be prime.

Correction
The correct wording of the end of this sentence is:
 * ... a Mersenne number whose index was a Mersenne prime would itself be prime.

This has been corrected in of $1997$.