Dirichlet
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Lejeune Dirichlet
lived from
1805 to 1859

Lejeune Dirichlet is best known for his proof that in any arithmetic progression with first term coprime to the difference there are infinitely many primes.

He also made contributions to Fermat's Last Theorem.

See more at:   http://mathshistory.st-andrews.ac.uk/Biographies/Dirichlet.html