The high-energy electrons interact with the Coulomb potential, which for a crystal can be considered in terms of a Fourier series (see for instance Ashcroft and Mermin), that is V ( r ) = ∑ ∑ V g exp ⁡ ⁡ ( 2 π π i g ⋅ ⋅ r ) with g a reciprocal lattice vector and V g the corresponding Fourier coefficient of the potential. The reciprocal lattice vector is often referred to in terms of Miller indices ( h k l ) , a sum of the individual reciprocal lattice vectors A , B , C with integers h , k , l in the form: g = h A + k B + l C (Sometimes reciprocal lattice vectors are written as a ∗ ∗ , b ∗ ∗ , c ∗ ∗ and see note.) The contribution from the V g needs to be combined with what is called the shape function (e.g.), which is the Fourier transform of the shape of the object. If, for instance, the object is small in one dimension then the shape function extends far in that direction in the Fourier transform—a reciprocal relationship.