Does the accented lower case C in French come from the Greek sigma?

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I assume you're referring to this letter: ç. The little hook under the c is actually called a cedilla (in French, cedille), not an accent.

No, it's unrelated to the Greek sigma. From Wikipedia: "The tail originated in Spain as the bottom half of a miniature cursive z." See these articles for more information.

https://en.wikipedia.org/wiki/Cedilla

https://en.wikipedia.org/wiki/%C3%87