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| In [[mathematics]], certain kinds of mistaken proof are often exhibited, and sometimes collected, as illustrations of a concept of '''mathematical fallacy'''. There is a distinction between a simple ''mistake'' and a ''mathematical fallacy'' in a proof: a mistake in a proof leads to an '''invalid proof''' just in the same way, but in the best-known examples of mathematical fallacies, there is some concealment in the presentation of the proof. For example, the reason validity fails may be a [[division by zero]] that is hidden by algebraic notation. There is a striking quality of the mathematical fallacy: as typically presented, it leads not only to an absurd result, but does so in a crafty or clever way.<ref>{{harvnb|Maxwell|1959|p=9}}</ref> Therefore these fallacies, for pedagogic reasons, usually take the form of spurious [[Mathematical proof|proofs]] of obvious [[contradiction]]s. Although the proofs are flawed, the errors, usually by design, are comparatively subtle, or designed to show that certain steps are conditional, and should not be applied in the cases that are the exceptions to the rules.
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| The traditional way of presenting a mathematical fallacy is to give an invalid step of deduction mixed in with valid steps, so that the meaning of [[fallacy]] is here slightly different from the [[Informal fallacy|logical fallacy]]. The latter applies normally to a form of argument that is not a genuine rule of logic, where the problematic mathematical step is typically a correct rule applied with a tacit wrong assumption. Beyond pedagogy, the resolution of a fallacy can lead to deeper insights into a subject (such as the introduction of [[Pasch's axiom]] of [[Euclidean geometry]]).<ref>{{harvnb|Maxwell|1959}}</ref> ''Pseudaria'', an ancient lost book of false proofs, is attributed to [[Euclid]].<ref>{{harvnb|Heath|Helberg|1908|loc=Chapter II, §I}}</ref>
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| Mathematical fallacies exist in many branches of mathematics. In [[elementary algebra]], typical examples may involve a step where division by zero is performed, where a [[root of a function|root]] is incorrectly extracted or, more generally, where different values of a [[multiple valued function]] are equated. Well-known fallacies also exist in elementary Euclidean geometry and [[calculus]].
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| ==Howlers==
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| Examples exist of ''mathematically '''correct''' results derived by '''incorrect''' lines of reasoning''. Such an argument, however [[truth|true]] the conclusion, is mathematically [[validity|invalid]] and commonly is known as a '''howler'''. Consider for instance the calculation:
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| :<math>\frac{16}{64} = \frac{16\!\!\!/}{6\!\!\!/4}=\frac{1}{4}.</math>
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| Although the conclusion 16/64 = 1/4 is correct, there is a fallacious, invalid cancellation in the middle step. Bogus proofs, calculations, or derivations constructed to produce a correct result in spite of incorrect logic or operations were termed ''howlers'' by Maxwell.<ref>{{harvnb|Maxwell|1959}}</ref> Outside the field of mathematics the term "''[[Howler (error)|howler]]''" has various meanings, generally less specific.
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| == Division by zero ==
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| The [[Division by zero|division-by-zero fallacy]] has many variants.
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| === All numbers equal all other numbers ===
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| The following example uses division by zero to "prove" that 2 = 1,
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| but can be modified to prove that any number equals any other number.
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| 1. Let ''a'' and ''b'' be equal non-zero quantities
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| :<math>a = b \,</math>
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| 2. Multiply through by ''a''
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| :<math>a^2 = ab \,</math>
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| 3. Subtract <math>b^2 \,</math>
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| :<math>a^2 - b^2 = ab - b^2 \,</math>
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| 4. [[Factorization|Factor]] both sides
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| :<math>(a - b)(a + b) = b(a - b) \,</math>
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| 5. Divide out <math>(a - b) \,</math>
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| :<math>a + b = b \,</math>
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| 6. Observing that <math>a = b \,</math>
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| :<math>b + b = b \,</math>
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| 7. Combine like terms on the left
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| :<math>2b = b \,</math>
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| 8. Divide by the non-zero ''b''
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| :<math>2 = 1 \,</math>
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| ''[[Q.E.D.]]''<ref>Harro Heuser: ''Lehrbuch der Analysis - Teil 1'', 6th edition, Teubner 1989, ISBN 978-3-8351-0131-9, page 51 (German).</ref>
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| The fallacy is in line 5: the progression from line 4 to line 5 involves division by ''a'' − ''b'', which is zero since ''a'' equals ''b''. Since [[division by zero]] is undefined, the argument is invalid.
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| == Multivalued functions ==
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| Many functions do not have a unique [[inverse function|inverse]]. For instance squaring a number gives a unique value, but there are two possible [[square root]]s of a positive number. The square root is [[multivalued function|multivalued]]. One value can be chosen by convention as the [[principal value]], in the case of the square root the non-negative value is the principal value, but there is no guarantee that the square root function given by this principal value of the square of a number will be equal to the original number, e.g. the square root of the square of −2 is 2. | |
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| ==Calculus==
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| Calculus as the mathematical study of infinitesimal change and [[limit of a function|limits]] can lead to mathematical fallacies if the properties of [[integrals]] and [[differential (mathematics)|differentials]] are ignored. For instance, [[integration by parts]] can be used to prove that 0 = 1.<ref>{{cite journal|first=Ed|last=Barbeau|journal=The College Mathematics Journal|volume=21|number=3|year=1990|pages=216–218|title=Fallacies, Flaws and Flimflam #19: Dolt's Theorem}}</ref> Letting {{math|''u'' {{=}} 1 / log ''x''}} and {{math|''dv'' {{=}} ''dx'' / ''x''}}, we may write:
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| : <math>\int \frac{1}{x \, \log x} \, dx = 1 + \int \frac{1}{x \, \log x} \, dx</math>
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| after which the antiderivatives may be cancelled yielding 0 = 1. The problem is that antiderivatives are only defined up to a constant and shifting them by 1 or indeed any number is allowed. The proof can be fixed by introducing the arbitrary integration limits ''a'' and ''b''.
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| : <math>\int_a^b \frac{1}{x \, \log x} \, dx = 1 |_a^b + \int_a^b \frac{1}{x \, \log x} \, dx = 0 + \int_a^b \frac{1}{x \, \log x} \, dx = \int_a^b \frac{1}{x \, \log x} \, dx</math>
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| Since the difference between two values of a constant function vanishes, the same definite integral appears on both sides of the equation.
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| ==Power and root==
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| Fallacies involving disregarding the rules of elementary arithmetic through an incorrect manipulation of the [[nth root|radical]]. For complex numbers the [[Exponentiation#Failure of power and logarithm identities|failure of power and logarithm identities]] has led to many fallacies.
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| === Positive and negative roots ===
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| Invalid proofs utilizing powers and roots are often of the following kind:<ref>{{harvnb|Maxwell|1959|loc=Chapter VI, §I.2}}</ref>
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| :<math>1 = \sqrt{1} = \sqrt{(-1)(-1)} = \sqrt{-1}\sqrt{-1}=i \cdot i = -1.</math>
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| The fallacy is that the rule <math>\sqrt{xy} = \sqrt{x}\sqrt{y}</math> is generally valid only if both ''x'' and ''y'' are positive, which is not the case here.
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| Although the fallacy is easily detected here, sometimes it is concealed more effectively in notation. For instance,<ref>{{harvnb|Maxwell|1959|loc=Chapter VI, §I.1}}</ref> consider the equation
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| :<math>\cos^2x=1-\sin^2x</math>
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| which holds as a consequence of the [[Pythagorean theorem]]. Then, by taking a square root,
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| :<math>\cos x = (1-\sin^2x)^{1/2}</math>
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| so that
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| :<math>1+\cos x = 1+(1-\sin^2x)^{1/2}.</math>
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| But evaluating this when ''x'' = π implies
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| :<math>1-1 = 1+(1-0)^{1/2}</math>
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| or
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| :<math>0=2</math>
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| which is incorrect.
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| The error in each of these examples fundamentally lies in the fact that any equation of the form
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| :<math>x^2 = a^2</math>
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| has two solutions, provided ''a'' ≠ 0,
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| :<math>x=\pm a</math>
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| and it is essential to check which of these solutions is relevant to the problem at hand.<ref>{{harvnb|Maxwell|1959|loc=Chapter VI, §II}}</ref> In the above fallacy, the square root that allowed the second equation to be deduced from the first is valid only when cos ''x'' is positive. In particular, when ''x'' is set to π, the second equation is rendered invalid. | |
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| Another example of this kind of fallacy, where the error is immediately detectable, is the following invalid proof that −2 = 2. Letting ''x'' = −2, and then squaring gives
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| :<math>x^2 = 4\,</math>
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| whereupon taking a square root implies
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| :<math>x = \sqrt{4} = 2,</math>
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| so that ''x'' = −2 = 2, which is absurd. Clearly when the square root was extracted, it was the ''negative'' root −2, rather than the ''positive'' root, that was relevant for the particular solution in the problem.
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| Alternatively, imaginary roots are obfuscated in the following:
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| :<math>\sqrt{-1} = (-1)^\frac{2}{4} = ((-1)^2)^\frac{1}{4} = 1^\frac{1}{4} = 1</math>
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| The error here lies in the last equality, where we are ignoring the other fourth roots of 1,<ref>In general, the expression <math>\sqrt[n]{1}</math> evaluates to ''n'' complex numbers, called the [[Root of unity|''n''th roots of unity]].</ref> which are −1, ''i'' and −''i'' (where ''i'' is the [[imaginary unit]]). Seeing as we have squared our figure and then taken roots, we cannot always assume that all the roots will be correct. So the correct fourth are ''i'' and −''i'', which are the imaginary numbers defined to be <math>\sqrt{-1}</math>.
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| === Complex exponents ===
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| When a number is raised to a complex power, the result is not uniquely defined (see [[Exponentiation#Failure of power and logarithm identities|Failure of power and logarithm identities]]). If this property is not recognized, then errors such as the following can result:
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| :<math>
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| \begin{align}
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| e^{2 \pi i} &= 1 \\
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| (e^{2 \pi i})^{i} &= 1^{i} \\
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| e^{-2 \pi} &= 1 \\
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| \end{align}
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| </math>
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| The error here is that the rule of multiplying exponents as when going to the third line does not apply unmodified with complex exponents, even if when putting both sides to the power {{math|''i''}} only the principal value is chosen. When treated as [[multivalued function]]s, both sides produce the same set of values, being {{math|1={e<sup>2''πn''</sup> {{!}} ''n'' ∈ ℤ} }}.
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| == Geometry ==
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| Many mathematical fallacies in [[geometry]] arise from using in an additive equality involving oriented quantities (such adding vectors along a given line or adding oriented angles in the plane) a valid identity, but which fixes only the absolute value of (one of) these quantities. This quantity is then incorporated into the equation with the wrong orientation, so as to produce an absurd conclusion. This wrong orientation is usually suggested implicitly by supplying an imprecise diagram of the situation, where relative positions of points or lines are chosen in a way that is actually impossible under the hypotheses of the argument, but non-obviously so. Such a fallacy is easy to expose by drawing a precise picture of the situation, in which some relative positions will be different form those in the provided diagram. In order to avoid such fallacies, a correct geometric argument using addition or subtraction of distances or angles should always prove that quantities are being incorporated with their correct orientation.
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| === Fallacy of the isosceles triangle ===
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| [[File:Isoscelesproof.svg|thumb|right]]
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| The fallacy of the isosceles triangle, from {{harv|Maxwell|1959|loc=Chapter II, § 1}}, purports to show that every [[triangle]] is [[isosceles triangle|isosceles]], meaning that two sides of the triangle are [[congruence (geometry)|congruent]]. This fallacy has been attributed to [[Lewis Carroll]].<ref>{{cite book| title=Lewis Carroll in Numberland| author=Robin Wilson| pages=169–170| publisher=Penguin Books| isbn=978-0-14-101610-8| year=2008}}</ref>
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| Given a triangle △ABC, prove that AB = AC:
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| # Draw a line [[bisection|bisecting]] ∠A
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| # Call the [[midpoint]] of line segment BC, D
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| # Draw the perpendicular bisector of segment BC, which contains D
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| # If these two lines are [[parallel (geometry)|parallel]], AB = AC; otherwise they intersect at point O
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| # Draw line OR perpendicular to AB, line OQ perpendicular to AC
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| # Draw lines OB and OC
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| # By [[solution of triangles|AAS]], △RAO ≅ △QAO (AO = AO; ∠OAQ ≅ ∠OAR since AO bisects ∠A; ∠ARO ≅ ∠AQO are both right angles)
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| # By [[solution of triangles|SAS]], △ODB ≅ △ODC (∠ODB,∠ODC are right angles; OD = OD; BD = CD because OD bisects BC)
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| # By [[congruence (geometry)|HL]],<ref>Hypotenuse-leg congruence</ref> △ROB ≅ △QOC (RO = QO since △RAO ≅ △QAO; BO = CO since △ODB ≅ △ODC; ∠ORB and ∠OQC are right angles)
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| # Thus, AR ≅ AQ, RB ≅ QC, and AB = AR + RB = AQ + QC = AC
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| ''Q.E.D.''
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| As a corollary, one can show that all triangles are equilateral, by showing that AB = BC and AC = BC in the same way.
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| All but the last step of the proof is indeed correct (those three pairs of triangles are indeed congruent). The error in the proof is the assumption in the diagram that the point O is ''inside'' the triangle. In fact, whenever AB ≠ AC, O lies ''outside'' the triangle. Furthermore, it can be shown that, if AB is longer than AC, then R will lie ''within'' AB, while Q will lie ''outside'' of AC (and vice versa). (Any diagram drawn with sufficiently accurate instruments will verify the above two facts.) Because of this, AB is still AR + RB, but AC is actually AQ − QC; and thus the lengths are not necessarily the same.
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| == Proof by induction ==
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| There exist several fallacious [[Proof by induction|proofs by induction]] in which the inductive step is mathematically correct but the basis case is not. Intuituvely, proofs by induction work by arguing that, if a statement is true in one case, it is true in the next case, and hence by repeatedly applying this it can be shown to be true for all cases. This proof shows that [[all horses are the same colour]].
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| # Let us say that any group of N horses is all of the same colour.
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| # If we remove a horse from the group, we have a group of N - 1 horses of the same colour. If we add another horse, we have another group of N horses. By our previous assumption, all the horses are of the same colour in this new group, since it is a group of N horses.
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| # Thus we have constructed two groups of N horses all of the same colour, with N - 1 horses in common. Since these two groups have some horses in common, the two groups must be of the same colour as each other.
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| # Therefore combining all the horses used, we have a group of N + 1 horses of the same colour.
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| # Thus if any N horses are all the same colour, any N + 1 horses are the same colour.
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| # This is clearly true for N = 1 (i.e. one horse is a group where all the horses are the same colour). Thus, by induction, N horses are the same colour for any positive integer N. i.e. all horses are the same colour.
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| The fallacy in this proof arises in line 3. For N = 1, the two groups of horses have N - 1 = 0 horses in common, and thus are not necessarily the same colour as each other, so the group of N + 1 = 2 horses is not necessarily all of the same colour. The argument works for all other N but fails in the basis case.
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| == See also ==
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| * [[List of incomplete proofs]]
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| * [[Paradox]]
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| * [[Proof by intimidation]]
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| ==Notes==
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| {{Reflist}}
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| ==References==
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| {{refbegin}}
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| * {{Citation | last1=Barbeau | first1=Edward J. | title=Mathematical fallacies, flaws, and flimflam | publisher=[[Mathematical Association of America]] | series=MAA Spectrum | isbn=978-0-88385-529-4 | id={{MathSciNet | id = 1725831}} | year=2000 |url=http://books.google.com/books?id=ugrXqDEyPU8C}}.
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| * {{Citation | last1=Bunch | first1=Bryan | title=Mathematical fallacies and paradoxes | publisher=[[Dover Publications]] | location=New York | isbn=978-0-486-29664-7 | id={{MathSciNet | id = 1461270}} | year=1997 |url=http://books.google.com/books?id=M6DvzoKlcicC}}.
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| * {{citation | first1=Sir Thomas Little|last1=Heath|first2=Johan Ludvig|last2=Heiberg|title=The thirteen books of Euclid's Elements, Volume 1|year=1908|publisher=The University Press}}.
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| * {{Citation | last1=Maxwell | first1=E. A. | title=Fallacies in mathematics | publisher=[[Cambridge University Press]] | id={{MathSciNet | id = 0099907}} | year=1959 |url=http://books.google.com/books?id=zNvvoFEzP8IC&printsec=frontcover |isbn=0-521-05700-0}}.
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| {{refend}}
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| == External links ==
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| * [http://www.cut-the-knot.org/proofs/index.shtml Invalid proofs] at [[Cut-the-knot]] (including literature references)
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| * [http://www.math.toronto.edu/mathnet/falseProofs/ Classic fallacies] with some discussion
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| * [http://www.ahajokes.com/math_jokes.html More invalid proofs from AhaJokes.com]
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| * [http://www.jokes-funblog.com/categories/49-Math-Jokes Math jokes including an invalid proof]
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| {{DEFAULTSORT:Mathematical Fallacy}}
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| [[Category:Proof theory]]
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| [[Category:Mathematical proofs]]
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| [[Category:Logical fallacies]]
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| [[Category:Humour]]
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