In mathematics and quantum mechanics, a Dirac operator is a differential operator that is a formal square root, or half-iterate, of a second-order operator such as a Laplacian. The original case which concerned Paul Dirac was to factorise formally an operator for Minkowski space, to get a form of quantum theory compatible with special relativity; to get the relevant Laplacian as a product of first-order operators he introduced spinors. It was first published in 1928. Formal definition In general, let D be a first-order differential operator acting on a vector bundle V over a Riemannian manifold M. If where ∆ is the Laplacian of V, then D is called a Dirac operator. In high-energy physics, this requirement is often relaxed: only the second-order part of D2 must equal the Laplacian. Examples Example 1 D = −i ∂x is a Dirac operator on the tangent bundle over a line. Example 2 Consider a simple bundle of notable importance in physics: the configuration space of a particle with spin confined to a plane, which is also the base manifold. It is represented by a wavefunction where x and y are the usual coordinate functions on R2. χ specifies the probability amplitude for the particle to be in the spin-up state, and similarly for η. The so-called spin-Dirac operator can then be written where σi are the Pauli matrices. Note that the anticommutation relations for the Pauli matrices make the proof of the above defining property trivial. Those relations define the notion of a Clifford algebra. Solutions to the Dirac equation for spinor fields are often called harmonic spinors. Example 3 Feynman's Dirac operator describes the propagation of a free fermion in three dimensions and is elegantly written using the Feynman slash notation. In introductory textbooks to quantum field theory, this will appear in the form where are the off-diagonal Dirac matrices , with and the remaining constants are the speed of light, being Planck's constant, and the mass of a fermion (for example, an electron). It acts on a four-component wave function , the Sobolev space of smooth, square-integrable functions. It can be extended to a self-adjoint operator on that domain. The square, in this case, is not the Laplacian, but instead (after setting ) Example 4 Another Dirac operator arises in Clifford analysis. In euclidean n-space this is where {ej: j = 1, ..., n} is an orthonormal basis for euclidean