The logo for Sigma pretty much looks like the letter E. The symbol has a deep bent in the middle towards the right and the middle ledge doesn't protrude as much as the top and bottom ledge.
This is the Sigma icon for Windows 10
. Here we go. Optimized for 32x32 px, these are the simplest icons in the line style. Downloadable formats are PNG and SVG, among others. Icons8 icon pack is unique in providing a huge number of icons designed by the same team of four designers who create the new icons every working day in the same style and with the same quality. Same cool as the Bat Face, huh.