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GuidePublished 6 Aug 2026Updated 13 Aug 20268 min readBy Kevin JoginComputational Number TheoryResourcesComputational AlgebraMacaulay2
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Computation and Authoritative Sources

Where to compute resolutions, Ext and cohomology — and why this collection does not reproduce tables of them.

  • Engineering
  • Mathematics
  • Part 1 of 1
  • 9 min read
  • KV-MATH-0172
Executive summary

Compute it, don't copy it

Resolutions, Ext modules, Betti tables and group cohomology rings are all computed rather than looked up, and the systems that compute them are actively maintained and versioned. This page records where the computations are done and which databases are authoritative. Consistent with the policy applied across this library, numerical and tabular results are not transcribed here — they carry a version dependency that a static page cannot honour.

Learning objectives

  • Identify the appropriate system for a given computation.
  • Locate the authoritative databases for group cohomology.
  • Understand why this collection carries method rather than tables.
  • Record the version and parameters needed to reproduce a computation.

Section 01Systems by task

Computational tools
TaskSystemNote
Free resolutions, Betti tables, Ext and Tor over polynomial ringsMacaulay2, SingularGröbner basis engines; graded Betti numbers are the standard output
Group cohomology rings of finite groupsGAP with the HAP package, MagmaMinimal resolutions over group algebras; ring structure and restriction maps
General ring-theoretic homological computationMagma, SingularBroad coverage including non-commutative cases
Lie algebra cohomologyGAP, LiE, custom Chevalley–Eilenberg codeFinite complexes make direct computation feasible
Simplicial and topological homologyGAP/HAP, CHomP, SageChain complexes from cell structures
Spectral sequence bookkeepingSage, custom codeDifferentials generally require human input
Formal verification of homological argumentsLean with mathlibAbelian categories, derived functors and Ext are formalised
Version and characteristic must be recorded

A Betti table depends on the base field, the characteristic, the term order and the software version. A computation quoted without those is not reproducible, and differences between characteristics are frequently the whole point. Record all four alongside any result.

Section 02Databases

Database

Group cohomology rings

Computed cohomology rings for the finite groups of small order are maintained as structured datasets alongside the systems that generated them. Consult the current release rather than a printed table.

Database

Small groups library

The classification of groups of small order, distributed with GAP and Magma, underpins any systematic cohomology computation.

Database

Betti tables and resolutions

Macaulay2 and Singular ship example libraries; published Betti tables should be regenerated rather than copied.

Section 03The sourcing policy

This collection carries the durable method layer: definitions, constructions, theorems, proof strategies and the reasoning that makes a computation correct. It deliberately does not reproduce computed tables — Betti numbers, cohomology ring presentations, resolution ranks — from any source.

  1. Stage 01ReproducibilityA computed table is valid only for a stated base field, characteristic and software version. A static page cannot carry that dependency reliably.
  2. Stage 02CurrencyDatabases are revised as algorithms improve and errors are found. A transcribed table ages silently, with no signal that it has become wrong.
  3. Stage 03IntegrityTranscription of dense numerical data introduces substitution errors that look plausible and are not detectable from the page itself.
  4. Stage 04ConsequenceMethod here; numbers from the live system, with the parameters recorded.
The rule applied throughout this library

Durable engineering and mathematical method belongs in the knowledge base. Numeric catalogue data belongs at its authoritative source, cited with the version that produced it. The same policy governs the Computational Algebraic Number Theory collection in this library.

ReferenceFrequently asked questions

Which system should I start with?

For commutative algebra and resolutions over polynomial rings, Macaulay2. For finite group cohomology, GAP with the HAP package. Both are freely available and widely used, so results are easy to have checked by others.

Why not include a table of small Ext computations?

Because the ones worth memorising are already derived in the relevant pages of this collection — Ext over ℤ between cyclic groups, for instance — and anything larger is version-dependent computed data that belongs at its source.

How should a computation be cited?

Record the system, the version, the base field and characteristic, the term order where relevant, and the exact input. That is enough for another person to reproduce it, which a transcribed result is not.

NavigateContinue in this stream

Curated next steps from this page. The site also surfaces algorithmically related reading below.

  • OrientationHomological Algebra: Discipline Overview
  • Derived FunctorsProjective and Injective Resolutions
  • ApplicationsModular Representation Theory
  • Cohomology of Lie AlgebrasHilbert's Syzygy Theorem

ProvenanceSources and further reading

This page is an original KEVOS explanatory article. It presents the underlying mathematics — definitions, algorithms, complexity results and selection criteria — in KEVOS editorial voice. No text is reproduced from any copyrighted source. Where numerical tables are relevant, KEVOS links to live authoritative databases rather than republishing static values.

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 Computation and Authoritative Sources. 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 Computation and Authoritative Sources as a network of definitions and implications, not as a list of formulas. The working vocabulary on this page—section, authoritative, resolutions, group, cohomology—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 Computation and Authoritative Sources?

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 section 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 — Number Theory I — Massachusetts Institute of Technology. Used for algebraic and analytic number theory. Accessed 2026-08-13.
  • MIT OpenCourseWare — Algebra I — Massachusetts Institute of Technology. Used for groups, vector spaces, linear transformations and linear groups. Accessed 2026-08-13.

On this page

  1. Executive summary
  2. Systems by task
  3. Databases
  4. The sourcing policy
  5. FAQ
  6. Continue in this stream
  7. Sources
Page ID
KV-MATH-0172
Taxonomy
ENG-MATH — Engineering / Mathematics
Collection
COL-HOMALG-001
Topic stream
HA-RESOURCES
Version
1.1.0 / content 2026.08
Last reviewed
2026-08-06

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