finite linear combinations) is dense in V. If V is a Hilbert space, an orthogonal basis is a total subset B of V such that elements in B are nonzero and pairwise orthogonal. Further, when each element in B has norm 1, then B is an orthonormal basis of V. Properties Let {bn} be a Schauder basis of a Banach space V over F = R or C. It is a subtle consequence of the open mapping theorem that the linear mappings {Pn} defined by are uniformly bounded by some constant C. When , the basis is called a monotone basis. The maps {Pn} are the basis projections. Let {b*n} denote the coordinate functionals, where b*n assigns to every vector v in V the coordinate αn of v in the above expansion. Each b*n is a bounded linear functional on V. Indeed, for every vector v in V, These functionals {b*n} are called biorthogonal functionals associated to the basis {bn}. When the basis {bn} is normalized, the coordinate functionals {b*n} have norm ≤ 2C in the continuous dual of V. A Banach space with a Schauder basis is necessarily separable, but the converse is false. Since every vector v in a Banach space V with a Schauder basis is the limit of Pn(v), with Pn of finite rank and uniformly bounded, such a space V satisfies the bounded approximation property. A theorem attributed to Mazur asserts that every infinite-dimensional Banach space V contains a basic sequence, i.e., there is an infinite-dimensional subspace of V that has a Schauder basis. The basis problem is the question asked by Banach, whether every separable Banach space has a Schauder basis. This was negatively answered by Per Enflo who constructed a separable Banach space failing the approximation property, thus a space without a Schauder basis. Examples The standard unit vector bases of c0, and of ℓp for 1 ≤ p < ∞, are monotone Schauder bases. In this unit vector basis {bn}, the vector bn in or in is the scalar sequence where all coordinates bn, j are 0, except the nth coordinate: where δn, j is the Kronecker delta. The space ℓ∞ is not separable, and therefore has no Schauder basis. Every orthonormal basis in a separable Hilbert space is a Schauder basis. Every countable orthonormal basis is equivalent to the standard unit vector basis in ℓ2. The Haar system is an example