# Where did the contour integral sign appear for the first time?

A simple question: Where did the contour integral sign appear for the first time?

Wikipedia says that it was introduced by physicist Arnold Sommefield in 1917 ( Table of mathematical symbols by introduction date ) but there are not any references to prove this

Does anyone know if this is true? Is there a scientific paper or a book where this symbol was introduced?

• My suspicion is that the notation was invented by Cauchy as he was the earliest mathematician to theorise the integral with 'complex limits' and Cauchys theorem very definitely uses a contour. Mathematicians being a conservative lot would have simply carried on with notation as such. However, this is my guess. I'd put it as a comment but for the moment my so-called smart phone isn't able to enter comments ... Aug 24 '20 at 2:00