Germain's interest in number theory was renewed when she read Carl Friedrich Gauss' monumental work . After three years of working through the exercises and trying her own proofs for some of the theorems, she wrote, again under the pseudonym of M. Le Blanc, to the author himself, who was one year younger than she. The first letter, dated 21 November 1804, discussed Gauss' and presented some of Germain's work on Fermat's Last Theorem. In the letter, Germain claimed to have proved the theorem for "n" = "p" − 1, where "p" is a prime number of the form "p" = 8"k" + 7. However, her proof contained a weak assumption, and Gauss' reply did not comment on Germain's proof.