John B. Cosgrave

Dr. John B. Cosgrave (born 5 January 1946) is an Irish mathematician specialising in number theory.

In January 1999, while preparing some work for his students, he identified a highly structured prime number with exactly two thousand digits.

Dubbing this prime a millennium prime, he wrote an email about it to a niece and nephew, which was subsequently published by Folding Landscapes,[2] the publishing house of the cartographer Tim Robinson.

He donated his author royalties to the Irish Cancer Society,[3] and subsequently wrote an Irishman's Diary[4] column about it for the Irish Times newspaper.

In July 1999 – while a participant in the Proth Search Group – he became the discoverer of the then-largest known composite Fermat number,[5] a record which his St. Patrick's College (Drumcondra) based Proth-Gallot Group twice broke in 2003, the 1999 record having stood until then.