Questions tagged [infinite-descent]

Infinite descent, also named Fermat's descent, is a technique to prove that a diophantine equation has no solutions by construction a 'smaller' solution from a given one.

Filter by
Sorted by
Tagged with
2
votes
1answer
35 views

Right angled triangles

Are right angled triangles the only triangles that can be divided into 2,4,8,16- - (2^n)smaller triangles, each being the same shape as the original? In other words is it the only one where infinite ...
7
votes
1answer
212 views

Does the equation $y^2=3x^4-3x^2+1$ have an elementary solution?

In this answer, I give an elementary solution of the Diophantine equation $$y^2=3x^4+3x^2+1.$$ In this post, the related equation $$y^2=3x^4-3x^2+1 \tag{$\star$}$$ is solved (in two different answers!)...
0
votes
1answer
48 views

Let there be $2n+1$ integers such that when one of them is removed, the sum can be divided into $2$ groups each with $n$ integers.

Let $a_1,a_2,\cdots,a_{2n+1}$ be integers such that, if any one of them is removed, the remaining ones can be divided into two sets of $n$ integers with equal sum. Prove $a_1 = a_2 = \cdots = a_{2n+1}$...
3
votes
0answers
73 views

Does it follow that if gcd$(x,6)=1$ and $n \ge 4$, then $x[3^{n-1}a + \sum\limits_{i=1}^{n-1}(3^{n-1-i}2^{P_i})]$ cannot have the same form?

Let: $n\ge 4, x, a, b$ be integers gcd$(c,d)$ be the greatest common divisor of $c$ and $d$ gcd$(x,6)=1$ $p_1, p_2, \dots, p_n$ be positive integers $q_1, q_2, \dots, q_n$ be positive integers $P_i = ...
4
votes
2answers
125 views

Collatz Conjecture: Is there a straightforward argument showing that there are no nontrivial 2 “step” repeats (where each “step” is an odd number)

Let: $C(x) = \dfrac{3x+1}{2^w}$ where $w$ is the highest power of $2$ that divides $3x+1$ $C_n(x) = C_1(C_2(\dots(C_n(x)\dots)) = \dfrac{3^n x + 3^{n-1} + \sum\limits_{i=1}^{n-1}3^{n-1-i}2^{\left(\...
1
vote
0answers
196 views

Infinite descent argument in the case of $x^5+y^5=z^5$.

Euler proved that: $$2u(u^2+3v^2)=(2a(a^2+3b^2))^3$$ had no non-trivial solutions. Several other mathematicians have proven that: $$x^5+y^5=z^5$$ Using a similar argument, it is easy to see when $x$ ...
3
votes
3answers
133 views

Proof Writing For Pythagorean Triple “Reduction” Problem

You are given a primitive Pythagorean triple $(a,b,c)$ such that $a,b,c$ are integers and $a<b<c$. “Reduce” the triple via the transformation $(a,b,c)\to (a,\sqrt{b^2-a^2},b)$, rearranging if ...
1
vote
0answers
53 views

How to prove that the difference of squares of two natural numbers can't be a perfect square if their sum is a perfect square? [duplicate]

I came across the following problem and wrote up the following proof a few years ago - what do you think? It is required to demonstrate the truth of Statement S1: "If $a^2 + b^2$ is a perfect square, ...
2
votes
0answers
33 views

Structured Theorems(Programs) in Number Theory Proving Logical Conjunctions Using Infinite Descent [closed]

Any comments/answers concerning the block-quote (red) text would be appreciated. I've been examining proofs of the Fundamental Theorem of Arithmetic and constructing my own using the method of ...
1
vote
0answers
68 views

Given integer $b \gt 1$ use infinite descent to prove that every integer $m \gt 0$ has one and only one Base-$b$ representation.

My motivation for looking into this was sparked by the 'big-list' question What are the principal (different) mechanisms of infinite descent proof? Is the following proof valid? If an integer $m \gt ...
3
votes
8answers
619 views

Combinatorics problems that can be solved via infinite descent

I'm looking for high school problems that can be solved with the method of infinite descent. Usually, those problems are from number theory, but I would be very happy if someone could provide a ...
3
votes
1answer
411 views

Lamé's proof of Fermat Last Theorem for n=3

Among the great mathematicians who struggled to try and figure out a proof of Fermat's last theorem in 19th century, Lamé was probably one of the most convinced in having succeded. I first came across ...
4
votes
2answers
158 views

Solving the homogeneous Diophantine equation $x^3 + 2y^3 = 7z^3$ for $x,y,z \in \mathbb{Q}$

I want to solve the homogeneous Diophantine equation $x^3 + 2y^3 = 7z^3$ for $x,y,z \in \mathbb{Q}$. First note that $(x,y,z) = (0,0,0)$ is a solution. For further solutions it suffices to search ...
8
votes
4answers
3k views

What is an example of a proof by minimal counterexample?

I was reading about proof by infinite descent, and proof by minimal counterexample. My understanding of it is that we assume the existance of some smallest counterexample $A$ that disproves some ...
-2
votes
1answer
160 views

How to solve $x^2+y^2=pz^2$ in $x,y,z\in\mathbb{N}$ if $p$ is such a prime that $p\equiv 3 \pmod 4$?

How to solve $x^2+y^2=pz^2$ in $x,y,z\in\mathbb{N}$ if $p$ is such a prime that $p\equiv 3 \pmod 4$? For such a prime we have lemma: $$p\mid a^2+b^2 \implies p\mid a\;\; {\rm and}\;\; p\mid b$$ Let'...
2
votes
3answers
635 views

Show that $x^2 + y^2 = 3$ has no rational points [duplicate]

Are there rational numbers such that $x^2 + y^2 = 3$ ? If I want to find a rational paramterizatio of $x^2 + y^2 = 1$ could start with the point $(1,0)$ and find lines $\ell$ of slope $m \in \...
7
votes
1answer
310 views

Geometric proof of number theory result: If the geometric and quadratic means of integers $a$ and $b$ are themselves integers, then $a=b$

We start with the following problem: Let $a$ and $b$ be positive integers such that their geometric and quadratic means are integers. Show that $a=b$. One possible approach is to write down the ...
0
votes
3answers
221 views

Find all $x$ such that $4^{27}+4^{1000}+4^{x}$ is a perfect square.

Let,$4^{27}+4^{1000}+4^{x}=n^2$ $4^{2}(4^{25}+4^{998}+4^{x-2})=n^2$ LHS is multiple of 4.$n$ is also multiple of 4 Let $n=4a _1$. $4^{2}(4^{25}+4^{998}+4^{x-2})=16.a_1^2$ $4^{25}+4^{998}+4^{x-2}=...
2
votes
2answers
355 views

Show $x^2$ + $y^2$ + $z^2$ = $2xyz$ has no non-trivial solutions using infinite descent.

I want to show that the equation $x^2$ + $y^2$ + $z^2$ = $2xyz$ has no non-trivial solutions using infinite descent. This question has been solved with infinite descent by showing that $x,y,z$ are ...
3
votes
2answers
70 views

Proving $x^2-y^2 = 2xyz$ has no integer solutions greater than 0.

"Prove that there are no integers $x,y, z >0$ such that $x^2-y^2=2xyz$. Solve by infinite descent." I've been able to solve this problem myself just by playing around with the algebra, but I have ...
1
vote
0answers
91 views

Proof By Descent FLT

So I've been studying the FLT and I am able to prove by descent that there are no solutions with $x,y,z$ natural numbers to $x^4+y^4=z^2$ I am trying to now prove by descent that there are no ...
0
votes
3answers
43 views

Proof of irrationality

I am trying to prove that $\,\sqrt[3]{2} \cdot (\sqrt[3]{2} + q)\,$ is irrational for all rational choice of $q$. However, I am completely stuck. I tried cubing it and trying the trick that you use ...
3
votes
2answers
163 views

Show that $\sqrt{2} \notin \mathbb{Q}(i)$ using infinite descent

Standard exercise is to show $\sqrt{2} \notin \mathbb{Q}$ (e.g. Wikipedia). There are examples on Math.SE such as [1, 2, 3, 4]. If we adjoin an element to $\mathbb{Q}$ does the same proof by ...
0
votes
2answers
168 views

How to understand the two definitions of finite descent are logical equivalent?

I find two definitions of finite descent principle. The first is in the book "A beginner's guide to mathematical logic", Ch4, P40: Suppose a property P is such that for any natural number n, if P ...
6
votes
6answers
1k views

Proving that there does not exist an infinite descending sequence of naturals using minimal counterexample

This might be a long-winded way of proving something so obvious, but I want to have it checked if it holds up. Claim: There does not exist a sequence $\{{a}_{n \in \mathbb{N}}\}$ of natural numbers ...
3
votes
2answers
543 views

Show that $x^3+2y^3+4z^3=0 $ has no non trivial solutions without using infinite descent

The question is to show that the equation $$x^3+2y^3+4z^3=0 $$ has no (non trivial) integer solutions. I know it can be done using infinite descent, but how do you do it using only the modular ...
93
votes
7answers
46k views

What is the algebraic intuition behind Vieta jumping in IMO1988 Problem 6?

Problem 6 of the 1988 International Mathematical Olympiad notoriously asked: Let $a$ and $b$ be positive integers and $k=\frac{a^2+b^2}{1+ab}$. Show that if $k$ is an integer then $k$ is a perfect ...
1
vote
0answers
115 views

Proofs by infinite descent on the number of prime factors of an integer

Many (most?) number theory proofs employing the method of infinite descent proceed something like this: Assume a given [Diophantine] equation (e.g., $x^3+y^3=z^3$) has solutions in positive integers. ...
3
votes
9answers
207 views

What are the principal (different) mechanisms of infinite descent proof?

I’m interested in building a list (including, where possible, links to proofs/papers/examples) which presents all known mechanisms of infinite descent (ID). I think this list would best be presented ...
4
votes
6answers
198 views

Find all solutions to the equation $x^2 + y^2 + xy = (xy)^2$

Can anyone help me find the number of solutions to the equation: $$ x^2 + y^2 + xy = (xy)^2 $$ Let me give a brief account of what I've tried to proceed with: Case 1: One of $x$ and $y$ is odd. ...
6
votes
3answers
142 views

Fermat's Challenge of composition of numbers

In his letter to Carcavi (August 1659), Fermat mentions the following challenge There is no number, one less than a multiple of $3$, composed of a square and the triple of another square. He ...
5
votes
3answers
743 views

Fermat's infinite descent for finding the squares that sum to a prime

Fermat's theorem on sum of two squares states that an odd prime $p = x^2 + y^2 \iff p \equiv 1 \pmod 4$ Applying the descent procedure I can get to $a^2 + b^2 = pc$ where $c \in \mathbb{Z} \gt 1$ I ...
3
votes
6answers
181 views

find all natural solution that satisfy $x^2+y^2 = 3z^2$

I need to find all natural $x,y,z$ that satisfy the following $x^2+y^2 = 3z^2$ $(0,0,0)$ is an answer of course. What I tried: I tried solving with congruences. I know that every square number is ...
3
votes
5answers
108 views

If $a,b,c$ are integers such that $4a^3+2b^3+c^3=6abc$, is $a=b=c=0$?

If $a,b,c$ are integers such that $4a^3+2b^3+c^3=6abc$ , then is it true that $a=b=c=0$ ? I was thinking of infinite descent but can't actually proceed , please help. Thanks in advance
6
votes
2answers
684 views

Why is this proof that $\sqrt{2}$ is irrational titled as “Proof by infinite descent”?

I am reading this wikipedea article on the proof of irrationality of $\sqrt{2}$. It uses the principle of infinite descent. I understand it as: We assume $\sqrt{2}=\dfrac pq$, where $p$ and $q$ are ...
3
votes
4answers
305 views

If $\gcd(a,b)=1$, then every odd factor of $a^2 + 3b^2$ has this same form

EDIT: Please see EDIT(2) below, thanks very much. I want to prove by infinite descent that the positive divisors of integers of the form $a^2+3b^2$ have the same form. For example, $1^2+3\cdot 4^2=49=...
11
votes
9answers
2k views

Solve $x^2+2=y^3$ using infinite descent?

just so this doesn't get deleted, I want to make it clear that I already know how to solve this using the UFD $\mathbb{Z}[\sqrt{-2}]$, and am in search for the infinite descent proof that Fermat ...
1
vote
0answers
116 views

Can Fermat's descent for $x^4+y^4=z^2$ be interpreted on a conic?

Fermat proved the Diophantine equation $$(x^2)^2 + (y^2)^2 = z^2$$ has only solutions $(0,0,0)$, $(0,\pm 1,\pm 1)$ and $(\pm 1,0,\pm 1)$ using "infinite descent". The conic $C : X^2 + Y^2 - 1$ has a ...
12
votes
1answer
457 views

How to get Fermat's descent working on the conic $x^4+y^4=2z^2$?

Fermat solved the Diophantine equation $(x^2)^2 + (y^2)^2 = z^2$ using descent, the key step was using the Pythagorean triples: $x^2 = u^2 - v^2$ $y^2 = 2 u v$ $z = u^2 + v^2$ but then it is seen ...