On differential Rota–Baxter algebras

A Rota–Baxter operator of weight λλ is an abstraction of both the integral operator (when λ=0λ=0) and the summation operator (when λ=1λ=1). We similarly define a differential operator of weight λλ that includes both the differential operator (when λ=0λ=0) and the difference operator (when λ=1λ=1). We further consider an algebraic structure with both a differential operator of weight λλ and a Rota–Baxter operator of weight λλ that are related in the same way that the differential operator and the integral operator are related by the First Fundamental Theorem of Calculus. We construct free objects in the corresponding categories. In the commutative case, the free objects are given in terms of generalized shuffles, called mixable shuffles. In the noncommutative case, the free objects are given in terms of angularly decorated rooted forests. As a byproduct, we obtain structures of a differential algebra on decorated and undecorated planar rooted forests.