Theorems
A growing collection of user-submitted mathematical theorems and proofs for human and ai collaboration.
Continuous Map Between Metric Spaces
definitiondef:continuous-map-metric-spaces-2026bAnalysisTopologyDefines continuity of a map from a subset of a metric space into a metric space, at a point relative to the subset and on the whole subset, by the epsilon-delta condition.Elementary Order Arithmetic in an Ordered Field
lemmalem:ordered-field-order-arithmetic-2026bAnalysisAlgebraElementary order rules in an ordered field: strict order is compatible with addition and with multiplication by positive elements, mixed transitivity, sign reversal, positivity of products, of 1 and of inverses, halving, the least of two elements, and order reversal of reciprocal…Existence and Uniqueness of the Nonnegative Square Root of a Nonnegative Real Number
theoremthm:real-nonnegative-square-root-2026aAnalysisLet be the set of real numbers, which by that definition is an ordered field, with order relation , in which every nonempty subset that is bounded above has a least upper bound. Write and for the additive and multiplicative identities of the underlying…- In a field, zero annihilates, signs multiply as usual, there are no zero divisors, the difference of squares and the square of a sum expand as usual, and negation is an involution with -(x-y)=y-x.
Existence and Uniqueness of the Positive Semi-Definite Square Root
theoremthm:positive-semidefinite-square-root-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector , and suppose that is finite-dimensional and . Let be a linear operator on that is self-adjoint and positive semi-definite. Then there is exactly…A Positive Semi-Definite Square Root Acts on Eigenvectors by the Nonnegative Square Root
lemmalem:psd-square-root-eigenvector-action-2026bAnalysisLinear AlgebraLet together with be a complex inner product space, and let be a linear operator on that is self-adjoint and positive semi-definite. Let be a real number with , the order being that of the ordered field of real numbe…An Operator with an Orthonormal Eigenbasis is Positive Semi-Definite Exactly When its Eigenvalues are Nonnegative
lemmalem:positive-semidefinite-iff-nonnegative-eigenvalues-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector . Let be a natural number, let be the initial segment determined by , and let be an -tuple in that is an orthonormal basis of , with comp…Operators Diagonal in an Orthonormal Basis
lemmalem:orthonormal-diagonal-operator-2026aAnalysisLinear AlgebraLet together with be a complex inner product space. Let be a natural number, let be the initial segment determined by , and let be an -tuple in that is an orthonormal basis of , with components . Let…The Eigenvalues of an Operator with an Orthonormal Eigenbasis
lemmalem:eigenvalues-orthonormal-eigenbasis-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector . Let be a natural number, let be the initial segment determined by , and let be an -tuple in that is an orthonormal basis of , with comp…Action of an Operator with an Orthonormal Eigenbasis
lemmalem:orthonormal-eigenbasis-action-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector . Let be a natural number, let be the initial segment determined by , and let be an -tuple in that is an orthonormal basis of , with comp…Integration by Parts for Indefinite Lebesgue Integrals on a Compact Interval
lemmalem:lebesgue-integration-by-parts-2026bAnalysisLet be a real number, and let be measurable with respect to the trace Borel -algebra on and Lebesgue integrable over . Let and be real numbers and define…Multivariate Taylor Expansion with Uniform Second-Order Remainder
lemmalem:taylor-second-order-uniform-2026bAnalysisLet be a natural number, let be an open subset of Euclidean space, and let be of class on (via clause 3 there); write for the partial derivative with respect to the -th coordinate. Let be…Continuous Real-Valued Functions on a Compact Interval are Bounded
lemmalem:continuous-compact-interval-bounded-2026bAnalysisLet and be real numbers with , and let be continuous on , the interval being regarded as a subset of the real line with the absolute value metric and carrying the same metric. Then there is a real number such that…Spectral Theorem for a Self-Adjoint Operator in Finite Dimensions
theoremthm:spectral-theorem-self-adjoint-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector , and suppose that is finite-dimensional and ; write for its dimension and let be the initial segment determined by . Let be a…A Self-Adjoint Operator on a Finite-Dimensional Space has a Unit Eigenvector
theoremthm:self-adjoint-eigenvalue-existence-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector , and suppose that is finite-dimensional and . Let be a linear operator on that is self-adjoint. Then there are a unit vector and a…Rayleigh Quotient of a Self-Adjoint Operator
definitiondef:rayleigh-quotient-2026aAnalysisLinear AlgebraLet together with be a complex inner product space, let be a linear operator on that is self-adjoint, and let be the set of unit vectors of . The Rayleigh quotient of is the map sending each to…Elementary Properties of a Self-Adjoint Operator
lemmalem:self-adjoint-elementary-properties-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector , and let be a linear operator on that is self-adjoint. Then the following hold. 1. (Real values on the diagonal) For every the complex number…- In an ordered field, 1 is nonnegative, sums of nonnegative elements are nonnegative, x is at most y exactly when y-x is nonnegative, inverses of positive elements are nonnegative, multiplication by a nonnegative element preserves order, and (x-y)^2 is at most 2(x^2+y^2).
The Orthogonal Complement of a Unit Vector
lemmalem:orthogonal-complement-unit-vector-2026aAnalysisLinear AlgebraLet together with be a complex inner product space with zero vector , and suppose that is finite-dimensional and ; write for its dimension. Let be a unit vector, let be the…The Orthogonal Complement of a Linear Subspace is a Linear Subspace
lemmalem:orthogonal-complement-is-subspace-2026aAnalysisLinear AlgebraLet together with be a complex inner product space and let be a linear subspace of . Then the orthogonal complement is a linear subspace of .