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The scaled real-column Householder procedure terminates with defined statuses
Statement
On every admitted finite input of Scaled Householder reflector formation and application, the formation and application procedure terminates with its specified identity, reflector or first-failure return. Each specified separate-block application also terminates with its output or first failure. The zero-column branch returns the identity action and the copied block without evaluating a reflector denominator. In a successful nonzero-column branch the stored vector is nonzero, so its associated exact real reflector is orthogonal and also acts unitarily on complex blocks. This statement asserts neither unconditional arithmetic success nor a backward-error bound for a successful call.
Facts & Assumptions
Given: The finite input dimensions and admitted arithmetic model of the procedure.
Scaled Householder reflector formation and application specifies the finite indexed scalar schedule, componentwise zero test, first-failure returns and positive-normal test on the computed squared norm of the stored vector.
Operationwise real and complex floating point arithmetic with rounded square roots and fused multiply-add makes each elementary operation return a finite value or its failure; exact-zero operations, copies and sign changes are exact.
Proof
The zero test scans a finite column. If it succeeds, [F1] returns its initialized identity record, zero target and copied finite block without forming any squared-vector norm or quotient. Otherwise the scaling maximum, the two norm sums, coefficient formation, column dots and ordered update products each use finitely many indices. By [F2], each attempted arithmetic operation returns its next value or a first failure. The extra finite positivity and normality tests either return failure or allow the next finite stage. Thus formation and application terminate with a declared record.
A specified separate block has finitely many columns and rows. Its sequential dots, coefficient-vector products and ordered update products have finite bounds; [F2] again makes each attempt return or fail. Hence this additional application terminates, including zero columns of the separate block.
In a successful nonzero-column branch, the stored squared-norm sum is positive by [F1]. If the stored vector were zero, every square and every sum in that computation would be exact zero by [F2], contradicting that positive test. Therefore and . For , direct multiplication gives , and . Its real entries give the same adjoint identity over . Together with steps 1.1–1.2 this proves every stated status and representative property.
Remarks
The reference entries are inherited from the analyzed procedure as method context. This result extracts and composes its explicit finite schedule and representative algebra; no verbatim external theorem or additional external reading is claimed.
This is a local extraction of finite-loop and representative arguments from the existing procedure carrier. Its error estimates remain in lem-scaled-householder-step-meets-local-backward-error-premise under that lemma's distinct hypotheses; no external full-text reading is claimed.
Depends on
Used by
Cited to discharge well-definedness by Scaled Householder reflector formation and application.
Dependency tree · two levels
5 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Fundamentals of Numerical Computation, floating-point arithmetic (standard reference, not scraped)
- Fundamentals of Numerical Computation, Householder triangularization (standard reference, not scraped)
- Fundamentals of Numerical Computation, QR factorization and least squares (standard reference, not scraped)
- Stuart and Voss, Matrix Analysis and Algorithms, Warwick lecture notes (standard reference, not scraped)