French mathematician, physicist, and philosopher of science (1854–1912) who famously formulated the ‘Poincare Conjecture,’ which went unsolved for nearly a century. Scarce ALS in French, signed “Poincare,” one page, 4.5 x 7, no date. Untranslated letter to social psychologist Gustave Le Bon, regretfully declining an invitation to attend a banquet due to sickness. In fine condition. Pre-certified PSA/DNA.
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