EXCLUSIVE! The mathematically savvy among us will know that the square root of -1 is I (for imaginary), that 2 cubed is 8, and that the Greek letter sigma means "sum up." The rest of us will remain blissfully in the dark. Navy T-shirt is preshrunk 100% cotton. Navy sweatshirt is 50/50 cotton-poly. Sizes M-XXL. Imported.