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 Sigma icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 5,700 icons, all in the same style and quality.