Symbols in calculus

• They follow rhos
• Frat letters
• Sum symbols in math
• Summation signs
• Letters from Greece
• Greek letters after rhos
• Odysseus trio, to Homer
• Greek letters

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sigma \Sig"ma\, n.; pl. Sigmas . [L., from Gr. ???, ???.] The Greek letter [Sigma], [sigma], or [sigmat] (English S, or s). It originally had the form of the English C.

Wiktionary Word definitions in Wiktionary
n. (plural of sigma English)