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A finite coefficient vector has a canonical polynomial representative
Statement
For every and reals , the function is the evaluation of a formal real polynomial of degree at most . The representative can be chosen so that it is the zero polynomial exactly when every ; if , it has degree and leading coefficient . Here the coefficient vector may be padded by zeros for the displayed sum; padding describes the evaluation, not a change to the formal polynomial object.
Facts & Assumptions
Given: A natural and a finite real coefficient vector .
Formal real polynomials, evaluation, degree, leading coefficient, and monic polynomials: a nonzero formal polynomial is a finite coefficient list whose final coefficient is nonzero; its evaluation is the corresponding finite sum, and the zero polynomial is a separate object.
Finite sums and finite products, by recursion and Laws of finite sums and finite products: the empty sum is zero, a finite sum extends by its last term, and the finite-sum notation agrees with that recursion.
The principle of mathematical induction: a property true at zero and preserved by successors holds for every natural number.
The reals form a field and Multiplication by zero: : real addition and multiplication have the field identities, including .
Proof
For , if choose the zero polynomial, whose evaluation is ; if , choose the one-term formal list , of degree and leading coefficient . In both cases its evaluation is .
Suppose the assertion holds for vectors of length , and consider . If , the finite-sum recursion and [F4] show that the displayed evaluation is the evaluation for the prefix vector; use its representative from the induction hypothesis. If , the formal coefficient list is valid by [F1], has degree and leading coefficient , and evaluates to the displayed sum by [F2]. The zero-characterization in the first case is inherited from the prefix; in the second case both the vector and the polynomial are nonzero.
Induction [F3] now supplies the representative for every . The construction also proves that it is zero exactly when all vector entries vanish, and that a nonzero final entry is its leading coefficient at degree .
Depends on
Used by
Dependency tree · two levels
28 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
- Mark Embree, Lecture Notes on Numerical Analysis, lecture 1 (standard reference, not scraped)
- Jean-Paul Berrut and Lloyd N. Trefethen, Barycentric Lagrange Interpolation (standard reference, not scraped)