Definition. Primitive: P ( A ) = { a ∈ A : ◻ a = a ⊗ 1 + 1 ⊗ a } {\displaystyle {\mathcal {P}}({\mathcal {A}})=\{a\in {\mathcal {A}}:\square a=a\otimes 1+1\otimes a\}} , where ◻ {\displaystyle \square } denotes the coproduct.
, where 
denotes the coproduct.
