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GuidePublished 7 Aug 2026Updated 13 Aug 20267 min readBy Kevin Jogin
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Engineering  /  Mathematics  — Polynomial Algorithms

Chinese Remaindering and Polynomial Interpolation

The Chinese remainder theorem for polynomials, and Lagrange interpolation as its special case.

Page KV-MATH-0446Reading time 3 minReviewed 2026-08-07Author Kevin Jogin

Executive summary

The Chinese remainder theorem holds for polynomials with pairwise coprime moduli, and taking the moduli to be distinct linear polynomials gives exactly Lagrange interpolation.

Recognising interpolation as a CRT instance explains why it is unique and why the formula takes the shape it does.

Learning objectives

  1. State the polynomial Chinese remainder theorem.
  2. Derive Lagrange interpolation as a special case.
  3. Compare the interpolation methods and their costs.

01The theorem

Theorem

Chinese remainder theorem for polynomials

For pairwise coprime h₁, ..., h_k with product H,

F[X]/(H) ≅ F[X]/(h₁) × ··· × F[X]/(h_k).

The construction of the isomorphism's inverse is identical to the integer case: form Hᵢ = H/hᵢ, invert it modulo hᵢ, and take the appropriate combination.

f = Σᵢ aᵢ · Hᵢ · (Hᵢ⁻¹ mod hᵢ)   mod H

02Interpolation as a special case

Take hᵢ = X − xᵢ for distinct points xᵢ. These are pairwise coprime, and a residue modulo X − xᵢ is a constant — namely the value of the polynomial at xᵢ.

Theorem

Lagrange interpolation

Given distinct points x₀, ..., x_n and values y₀, ..., y_n, there is a unique polynomial of degree at most n with f(xᵢ) = yᵢ, given by

f(X) = Σᵢ yᵢ ∏_{j ≠ i} (X − xⱼ)/(xᵢ − xⱼ).

Note
The uniqueness is the Chinese remainder theorem's uniqueness statement, and the formula is its explicit inverse. The product term is exactly Hᵢ scaled by its inverse modulo hᵢ, which for a linear modulus is a constant.

03Interpolation methods

  1. Lagrange formO(n²)Explicit; poor for adding a new point
  2. Newton formO(n²)Incremental; adding a point costs O(n)
  3. Fast interpolationO(n log² n)Subproduct tree; the asymptotic method
  4. Vandermonde solveO(n³)Never do this — it is the same problem stated worse
Interpolation method comparison
MethodAdding a pointNumerical behaviour
LagrangeRecompute everythingPoor over the reals
NewtonOne extra divided differenceBetter over the reals
FastRecompute the treeExact over finite fields
Caution
Solving a Vandermonde system by general Gaussian elimination costs cubic time for a problem that is inherently quadratic and admits an O(n log² n) method. It is a common and avoidable inefficiency.

Over finite fields the numerical column is irrelevant and the choice is purely about cost and incrementality. Newton form is the practical default; the fast method matters only for large degree.

04Frequently asked questions

Why must the points be distinct?

Because the moduli X − xᵢ must be pairwise coprime, which fails when two points coincide. Repeated points correspond to derivative conditions and give Hermite interpolation instead.

Does interpolation work over any field?

Yes, provided enough distinct points exist. Over F_q there are only q elements, so interpolating a degree-n polynomial requires n + 1 ≤ q — a real constraint for small fields.

How does this connect to secret sharing?

Directly. Shamir's scheme distributes evaluations of a random polynomial; any k shares interpolate it and recover the constant term, while fewer determine nothing. The threshold is exactly the interpolation requirement.

Related pages

  • The Chinese Remainder Theorem
  • Polynomial Modular Inverses
  • Mutual Independence and Secret Sharing

Sources and method

Structural reference: Victor Shoup, A Computational Introduction to Number Theory and Algebra, Version 1, Cambridge University Press, 2005 — book pages 406-408.

This page carries the durable method layer only: definitions, constructions, algorithms, complexity results and selection criteria, authored originally for KEVOS. No text is transcribed or paraphrased from the source, and no numeric tables or benchmark data are reproduced — these are routed to live authoritative sources instead.

Author: Kevin Jogin. Last reviewed 2026-08-07.

Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Chinese Remaindering and Polynomial Interpolation. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to turn a compact mathematical statement into a usable chain of definitions, claims, examples and checks. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Treat Chinese Remaindering and Polynomial Interpolation as a network of definitions and implications, not as a list of formulas. The working vocabulary on this page—interpolation, chinese, theorem, special, case—should be made explicit before any proof or computation begins. Record the ambient set or structure, the permitted operations and the equality or equivalence relation in use. A compact theorem often changes meaning when the base field, finiteness condition, commutativity assumption or direction of an action changes.

For a proof, write the hypotheses as a checklist and mark the line at which each one is used. For a computation, state the representation of the input, the arithmetic model, the termination condition and the output invariant. For a classification problem, distinguish existence from uniqueness and distinguish an object from its representation. These separations prevent a correct local calculation from being mistaken for the general result.

A useful worked example should be small enough to inspect completely but rich enough to exercise the main mechanism. Compute the result in two ways where practical: symbolically and by substitution, structurally and numerically, or directly and through a normal form. Then include one near-miss example in which a hypothesis fails. The contrast explains why the theorem is shaped as it is and gives the reader a diagnostic pattern for later problems.

Verification is part of the mathematics. Check domains and codomains, substitute proposed solutions, test identity and zero cases, compare dimensions or cardinalities, and confirm that maps respect the required operations. In numerical work, report precision, conditioning and a residual rather than digits alone. In algorithmic work, separate mathematical correctness from implementation complexity and resource limits.

Step-by-step operating method

  1. Fix the setting. State the objects, ambient structure, notation and assumptions before manipulating symbols.
  2. Separate claims. Distinguish definitions, hypotheses, conclusions, equivalent conditions and consequences.
  3. Choose a method. Select proof, construction, calculation or algorithm according to the question actually asked.
  4. Work a small case. Use the smallest non-trivial example to expose the mechanism and test edge behaviour.
  5. Verify independently. Substitute back, check invariants, test boundary cases or use an alternative derivation.

Worked-example protocol

Illustrative method—not a source theorem. Start with a small admissible input and list the definitions it must satisfy. Carry out each transformation on a separate line, citing the property that permits it. Preserve exact values until approximation is necessary. At the end, verify the output against the original definition and one invariant such as dimension, degree, determinant, order, norm or residual. Then alter one hypothesis and observe which step ceases to be valid. This protocol creates a reusable example without inventing a theorem-specific numerical answer.

StageRecordQuality check
InputObjects, domain, notation, assumptionsEvery symbol is defined
MethodPermitted operation or cited result at each stepAll hypotheses hold
OutputExact result and representationCorrect type, domain and form
VerificationSubstitution, invariant or alternative derivationIndependent agreement
Boundary testZero, identity, degenerate or failed hypothesisScope is understood

Common failure modes and recovery actions

1. Watch for

Using a theorem without checking every hypothesis.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Treating a suggestive example as a proof of the general case.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Changing notation or conventions part-way through an argument.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Hiding a division-by-zero, convergence, finiteness or commutativity assumption.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Reporting a computed result without a residual, substitution or structural check.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

  • Can every symbol be traced to a definition or prior result?
  • Which hypothesis does each major step use?
  • Does the method cover zero, identity, degenerate and boundary cases?
  • Can the conclusion be checked by a second representation or calculation?
  • Are mandatory requirements distinguished from recommendations and illustrative values?
  • Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
  • Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
  • Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Chinese Remaindering and Polynomial Interpolation?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

Which assumption about interpolation would change the result most if it proved false?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

What evidence would allow an independent reviewer to reproduce or challenge the conclusion?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

Which boundary, exception or failure case has not yet been tested?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

What must be handed over, monitored or reviewed after the immediate work is complete?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

Authoritative references and use notes

The sources below were selected as institutional or primary guidance for the broader practice. They support the handbook method; they do not imply that every statement or clause in a source applies to every project. Confirm the current edition, jurisdiction, contract and application before treating any requirement as mandatory.

  • MIT OpenCourseWare — Algebra I — Massachusetts Institute of Technology. Used for groups, vector spaces, linear transformations and linear groups. Accessed 2026-08-13.
  • The Stacks Project — table of contents — The Stacks Project. Used for commutative algebra, homological algebra, modules and derived categories. Accessed 2026-08-13.

Continue learning

Polynomial Modular InversesGuide · Engineering MathematicsNEXT LESSON →Mutual Independence and Secret SharingGuide · Engineering MathematicsEuclid's Algorithm for PolynomialsGuide · Engineering MathematicsSpeeding Up Polynomial Algorithms via Modular ComputationGuide · Engineering Mathematics
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