Template:Today in Math History for February 13

Calendar of Today in Math History for February
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• February 13, 1742 Christian Goldbach responds to Leonhard Euler's letter about ${\frac {2^{i}+2^{-i}}{2}}\approx {\frac {10}{13}}$ with his estimation that $2<x<3$ in $2^{xi}+2^{-xi}=0$.
• February 13, 1805 Lejeune Dirichlet is born in Düren.