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Showing 1–30 of 360 reference entries

Mathematics & logicExamples and counterexamples

Learn what a claim covers, explain why it works, and recognise an allowed case that shows it needs changing.

Method
Mathematics & logicCounting and zero

Count each object once, explain the final count, and distinguish quantity, position and identifying labels. Make zero’s role explicit.

Definition
Mathematics & logicNumber lines and order

Use number-line positions to compare whole numbers. Read the scale, recognise equality, and distinguish numerical distance from drawing size.

Definition
Mathematics & logicAddition and subtraction

Combine separate counts, remove a part, and find differences or missing parts. Connect objects, equations and number-line moves, then use tens and ones to regroup without changing value.

Definition
Mathematics & logicMultiplication and division

Count equal groups, distinguish sharing from making groups, and check with multiplication. Split calculations into parts; interpret zero and remainders in context.

Definition
Mathematics & logicFractions

Choose a whole, count equal fractional units, and connect sharing with division. Place fractions on the number line, including values beyond one, and explain what their numerator and denominator mean.

Definition
Mathematics & logic · Physics · MeasurementRatios and rates

Distinguish part-to-part ratios, fractions of a whole and rates with units. Scale quantities together, compare unit rates, and explain when a relationship is proportional. Check zero cases, starting amounts and whether a result fits its context.

Definition
Mathematics & logic · Physics · MeasurementQuantities and units

Describe a quantity as a numerical value with a unit, and distinguish its physical kind from its dimension. Build length, area, volume and rate units, check meaningful combinations, and separate exact unit relations from measurement uncertainty.

Definition
Mathematics & logic · Physics · MeasurementUnit conversions

Convert quantities with exact unit ratios, track cancellation through compound units, and apply the correct powers to area and volume. Distinguish temperature readings from differences, check dimensions and context, and retain measurement uncertainty when reporting.

Method
Mathematics & logic · Physics · MeasurementEstimation and uncertainty

Distinguish exact model values, rounded approximations and estimates from measurements. Declare the rounding rule, preserve information through calculations and unit changes, and interpret resolution, precision, accuracy and the stated uncertainty type without turning a report’s digits into guaranteed bounds.

Definition
Mathematics & logicVariables and expressions

Use symbols to name numbers or quantities, describe their allowed values, and evaluate expressions by replacing every occurrence consistently. Preserve grouping, signs, fractions and units, and distinguish a calculated value from solving an equation or proving two rules equivalent.

Definition
Mathematics & logicEquality and equations

Read equality as a relationship, find the permitted values that make an equation true, and justify one-step solutions by reversible operations on both sides. Work exactly with negative numbers and fractions, recognize all-solution and no-solution cases, and check the original equation, units and context.

Definition
Mathematics & logic · PhysicsCoordinates and tables

Locate a point from an ordered pair, read named axes and their scales, and connect the same values in words, tables and plots. Keep signs, fractions and units exact, distinguish an axis point from a quadrant point, and separate selected observations from a stated rule and its domain before drawing connections.

Definition
Mathematics & logic · PhysicsSlope and straight lines

Read a straight line as a stated relationship, calculate its slope from consistent signed coordinate changes, and interpret its intersections with the axes. Keep horizontal, vertical and coincident-point cases distinct, attach units and domain conditions to context models, and use numerical scales rather than appearance alone to judge a line’s change.

Definition
Mathematics & logic · Physics · MeasurementReading graphs

Read a graph’s quantities, units, ticks and context before judging its visual impression. Calculate differences, reference-based percentages and interval rates from the represented values; distinguish legitimate zoom from distorted spacing; and keep samples, interpolation, missing information, uncertainty and causal explanations separate.

Method
Mathematics & logic · PhysicsApproximations

Build a useful estimate, compare a correction with a stated reference scale, and check what is lost when you leave out a term.

Approximation
Mathematics & logic · PhysicsSymmetry and invariance

Specify a transformation, identify what stays unchanged, and distinguish symmetry of an object from invariance of a physical model.

Mathematical statement
Mathematics & logic · PhysicsVectors

Describe displacement with fixed-axis components, combine compatible vectors, and use small exact examples to introduce span, independence, basis and real vector spaces.

Definition
Mathematics & logic · InformationClassical propositions and connectives

Classical logic combines truth-valued propositions using defined connectives.

Definition
Mathematics & logicAxioms, deduction and mathematical proof

A mathematical proof derives a conclusion within stated assumptions and rules.

Method
Mathematics & logic · InformationSets, membership and basic operations

A set specifies membership without counting duplicate listings.

Definition
Mathematics & logic · InformationFunctions, domain and codomain

A function assigns exactly one output to each input in its domain.

Definition
Mathematics & logic · InformationMathematical induction

A base case and general successor step establish a predicate on all nonnegative integers.

Method
Mathematics & logic · MeasurementEuclidean right triangles and the Pythagorean theorem

In a Euclidean right triangle, squared leg lengths sum to the squared hypotenuse length.

Mathematical statement
Mathematics & logic · PhysicsSine, cosine and the unit circle

Sine and cosine are unit-circle coordinates whose squared values sum to one.

Mathematical statement
Mathematics & logic · InformationComplex numbers

A complex number has real coordinates a and b and an imaginary unit satisfying i²=−1.

Definition
Mathematics & logic · Information · PhysicsLinear maps and matrix representation

A linear map preserves linear combinations and has a matrix once bases are chosen.

Definition
Mathematics & logicLimit of a real function

A finite limit controls output error through sufficiently small input distance.

Definition
Mathematics & logic · PhysicsDerivative at a point

A derivative is a finite limit of local difference quotients.

Definition
Mathematics & logic · PhysicsDefinite integral and signed accumulation

A definite integral is the limiting signed accumulation of small contributions.

Definition
MethodSource checked

Examples and counterexamples

Mathematics & logic

Learn what a claim covers, explain why it works, and recognise an allowed case that shows it needs changing.

Key knowledge

A claim that P(x) holds for every x in a specified domain is false if an element of that domain makes P(x) false. Testing some successful examples does not establish the claim for an infinite domain.

Values and equations

¬(∀x ∈ D, P(x)) ⇔ ∃x ∈ D, ¬P(x)

D
Domain of the claim
P(x)
Predicate asserted of x

Conditions and scope

  • The counterexample must satisfy every hypothesis and belong to the stated domain.
  • This concerns universal mathematical claims, not a statistical claim about typical outcomes.
Sources · 7

References

Sources retain their recorded scope. A source link alone does not establish verification of every statement.

  1. University of Cambridge2 sources
    1. NRICH · Generalising and ProofUniversity of CambridgeTeaching resourcenrich.maths.orgRead source
    2. NRICH · On the Importance of PedantryUniversity of CambridgeTeaching resourcenrich.maths.orgRead source
  2. Ganita Prakash · Grade 6 · Patterns in MathematicsNCERTTextbook chapter · PDFncert.nic.inRead source
  3. Massachusetts Institute of Technology2 sources
    1. Mathematics for Computer Science (2015), §1.1 PropositionsEric Lehman, F. Thomson Leighton and Albert R. Meyer; MIT OpenCourseWareTextbook chapter · PDFocw.mit.eduRead source
    2. Mathematics for Computer Science, Spring 2015Eric Lehman, F. Thomson Leighton and Albert R. Meyer / MIT OpenCourseWareTextbook chapter · PDFocw.mit.eduRead source
  4. Contemporary Mathematics, 2.1: Statements and quantifiersOpenStaxTextbook chapteropenstax.orgRead source
  5. TIMES · Sets and Venn diagramsAMSI and University of Melbourne, ICE-EMTeaching resourceamsi.org.auRead source

Evidence and review

Universal-claim refutation and the domain requirement.

2026-10-10 · Checked the mathematical counterexample rule; excluded the source's informal generalisations about scientific induction.

Foundational review

2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.

Sources from other places

Scope or convention differs

A counterexample refutes an all-members claim in the stated set.

The ostrich example illustrates refutation; it does not independently prove every formal quantifier qualification.

Related articles

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Record identity
counterexamples
Revision
reference-core.1
Published lesson seed
orientation-reasoning