German mathematician (1845-1918) who created set theory, which has become a fundamental theory in mathematics. Cantor established the importance of one-to-one correspondence between the members of two sets, defined infinite and well-ordered sets, and proved that the real numbers are more numerous than the natural numbers. Very rare ALS in German, signed "G. Cantor," penned on a 5.5 x 3.5 postal card, February 18, 1906. Addressed from Halle, Germany, a short handwritten letter of thanks to important French mathematician Charles-Ange Laisant, in full (translated): "Many thanks for having sent me your essay on the mathematical initiation." Addressed on the reverse by Cantor. In fine condition, with a few small stains. Cantor remains extremely rare across all signed formats, with this letter representing our very first offering.
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