The physicist Arnold Sommerfeld was the first person to use the contour integral sign ∮ in his 1917 paper "Drude's dispersion theory from the point of view of Bohr's model and the constitution of H₂, O₂ and N₂" pic.twitter.com/jiX0Dho5pI
— Fermat's Library (@fermatslibrary) September 14, 2020