In algebra, a primitive element of a co-algebra C is an element x that satisfies μ = x ⊗ g + g ⊗ x {\displaystyle \mu =x\otimes g+g\otimes x} where μ {\displaystyle \mu } is the co-multiplication and g is an element of C that maps to the multiplicative identity 1 of the base field under the co-unit. Wikipedia