12nd  Differential_calculus">Top calculus topics: Differential calculus 
In mathematics, a differential operator is an operator defined as a function of the differentiation operator. It is helpful, as a matter of notation first, to consider differentiation as an abstract operation, accepting a function and returning another (in the style of a higherorder function in computer science).
There are certainly reasons not to restrict to linear operators; for instance the Schwarzian derivative is a wellknown nonlinear operator. Only the linear case will be addressed here.
Contents 
The most commonly used differential operator is the action of taking the derivative itself. Common notations for this operator include:
First derivatives are signified as above, but when taking higher, nth derivatives, the following alterations are useful:
For a function f of an argument x, the derivative operator is sometimes given:
The D notation's use and creation is credited to Oliver Heaviside, who considered differential operators of the form
in his study of differential equations.
One of the most frequently seen differential operators is the Laplacian operator, defined by
Another differential operator is the Θ operator, or theta operator, defined by^{[1]}
This is sometimes also called the homogeneity operator, because its eigenfunctions are the monomials in z:
In n variables the homogeneity operator is given by
As in one variable, the eigenspaces of Θ are the spaces of homogeneous polynomials.
Given a linear differential operator T
the adjoint of this operator is defined as the operator T ^{*} such that
where the notation is used for the scalar product or inner product. This definition therefore depends on the definition of the scalar product.
In the functional space of square integrable functions, the scalar product is defined by
If one moreover adds the condition that f or g vanishes for and , one can also define the adjoint of T by
This formula does not explicitly depend on the definition of the scalar product. It is therefore sometimes chosen as a definition of the adjoint operator. When T ^{*} is defined according to this formula, it is called the formal adjoint of T.
A (formally) selfadjoint operator is an operator equal to its own (formal) adjoint.
If Ω is a domain in R^{n}, and P a differential operator on Ω, then the adjoint of P is defined in L^{2}(Ω) by duality in the analogous manner:
for all smooth L^{2} functions f, g. Since smooth functions are dense in L^{2}, this defines the adjoint on a dense subset of L^{2}: P^{*} is a denselydefined operator.
The SturmLiouville operator is a wellknown example of formal selfadjoint operator. This secondorder linear differential operator L can be written in the form
This property can be proven using the formal adjoint definition above.
This operator is central to SturmLiouville theory where the eigenfunctions (analogues to eigenvectors) of this operator are considered.
Differentiation is linear, i.e.,
where f and g are functions, and a is a constant.
Any polynomial in D with function coefficients is also a differential operator. We may also compose differential operators by the rule
Some care is then required: firstly any function coefficients in the operator D_{2} must be differentiable as many times as the application of D_{1} requires. To get a ring of such operators we must assume derivatives of all orders of the coefficients used. Secondly, this ring will not be commutative: an operator gD isn't the same in general as Dg. In fact we have for example the relation basic in quantum mechanics:
The subring of operators that are polynomials in D with constant coefficients is, by contrast, commutative. It can be characterised another way: it consists of the translationinvariant operators.
The differential operators also obey the shift theorem.
The same constructions can be carried out with partial derivatives, differentiation with respect to different variables giving rise to operators that commute (see symmetry of second derivatives).
In differential geometry and algebraic geometry it is often convenient to have a coordinateindependent description of differential operators between two vector bundles. Let E and F be two vector bundles over a manifold M. An linear mapping of sections is said to be a kthorder linear differential operator if it factors through the jet bundle . In other words, there exists a linear mapping of vector bundles
such that
where denotes the map induced by on sections , and is the canonical (or universal) kthorder differential operator.
This just means that for a given sections s of E, the value of P(s) at a point is fully determined by the kthorder infinitesimal behavior of s in x. In particular this implies that P(s)(x) is determined by the germ of s in x, which is expressed by saying that differential operators are local. A foundational result is the Peetre theorem showing that the converse is also true: any local operator is differential.
An equivalent, but purely algebraic description of linear differential operators is as follows: an linear map P is a kthorder linear differential operator, if for any k + 1 smooth functions we have
Here the bracket is defined as the commutator
This characterization of linear differential operators shows that they are particular mappings between modules over a commutative algebra, allowing the concept to be seen as a part of commutative algebra.
