Can any improvements be made to my proof that “$sqrt{3} $ is irrational”?
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Suppose, for contradiction that $sqrt{3}$ is rational. Then there exists $a,b in mathbb{Z}$ such that $$frac{a}{b}= sqrt{3},$$
where $a/b$ is in its simplest form. Then the above equation implies $$a^2=3b^2.$$ If $b$ is even, then $a$ is even, which is a contradiction since $a/b$ is therefore not in its simplest form.
Now, consider $b$ to be odd, then a is odd. Then for $m,n in mathbb{Z}$, we have $$(2m+1)^2=3(2n+1)^2\ 4m^2+4m+1=12n^2+12n+3\
2(2m^2+2m)=2(6n^2+6n+1)\2(m^2+m)=2(3n^2+3n)+1.$$
The LHS is even since $m^2+m in mathbb{Z}$ and the RHS is odd since $ 3n^2+3nin mathbb{Z}$. This is a contradiction, and we therefore conclude that $sqrt{3}$ is irrational.
proof-verification proof-writing
$endgroup$
add a comment |
$begingroup$
Suppose, for contradiction that $sqrt{3}$ is rational. Then there exists $a,b in mathbb{Z}$ such that $$frac{a}{b}= sqrt{3},$$
where $a/b$ is in its simplest form. Then the above equation implies $$a^2=3b^2.$$ If $b$ is even, then $a$ is even, which is a contradiction since $a/b$ is therefore not in its simplest form.
Now, consider $b$ to be odd, then a is odd. Then for $m,n in mathbb{Z}$, we have $$(2m+1)^2=3(2n+1)^2\ 4m^2+4m+1=12n^2+12n+3\
2(2m^2+2m)=2(6n^2+6n+1)\2(m^2+m)=2(3n^2+3n)+1.$$
The LHS is even since $m^2+m in mathbb{Z}$ and the RHS is odd since $ 3n^2+3nin mathbb{Z}$. This is a contradiction, and we therefore conclude that $sqrt{3}$ is irrational.
proof-verification proof-writing
$endgroup$
7
$begingroup$
"Suppose, for contradiction that $sqrt{3}$ is rational."
$endgroup$
– John
Dec 20 '18 at 18:34
2
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What a silly error. Thanks.
$endgroup$
– user503154
Dec 20 '18 at 18:37
1
$begingroup$
Pretty slick. As a corollary (or a lemma toward), no square is of the form $4k+3$. Rather cute.
$endgroup$
– fleablood
Dec 20 '18 at 19:29
add a comment |
$begingroup$
Suppose, for contradiction that $sqrt{3}$ is rational. Then there exists $a,b in mathbb{Z}$ such that $$frac{a}{b}= sqrt{3},$$
where $a/b$ is in its simplest form. Then the above equation implies $$a^2=3b^2.$$ If $b$ is even, then $a$ is even, which is a contradiction since $a/b$ is therefore not in its simplest form.
Now, consider $b$ to be odd, then a is odd. Then for $m,n in mathbb{Z}$, we have $$(2m+1)^2=3(2n+1)^2\ 4m^2+4m+1=12n^2+12n+3\
2(2m^2+2m)=2(6n^2+6n+1)\2(m^2+m)=2(3n^2+3n)+1.$$
The LHS is even since $m^2+m in mathbb{Z}$ and the RHS is odd since $ 3n^2+3nin mathbb{Z}$. This is a contradiction, and we therefore conclude that $sqrt{3}$ is irrational.
proof-verification proof-writing
$endgroup$
Suppose, for contradiction that $sqrt{3}$ is rational. Then there exists $a,b in mathbb{Z}$ such that $$frac{a}{b}= sqrt{3},$$
where $a/b$ is in its simplest form. Then the above equation implies $$a^2=3b^2.$$ If $b$ is even, then $a$ is even, which is a contradiction since $a/b$ is therefore not in its simplest form.
Now, consider $b$ to be odd, then a is odd. Then for $m,n in mathbb{Z}$, we have $$(2m+1)^2=3(2n+1)^2\ 4m^2+4m+1=12n^2+12n+3\
2(2m^2+2m)=2(6n^2+6n+1)\2(m^2+m)=2(3n^2+3n)+1.$$
The LHS is even since $m^2+m in mathbb{Z}$ and the RHS is odd since $ 3n^2+3nin mathbb{Z}$. This is a contradiction, and we therefore conclude that $sqrt{3}$ is irrational.
proof-verification proof-writing
proof-verification proof-writing
edited Dec 20 '18 at 18:37
asked Dec 20 '18 at 18:32
user503154
7
$begingroup$
"Suppose, for contradiction that $sqrt{3}$ is rational."
$endgroup$
– John
Dec 20 '18 at 18:34
2
$begingroup$
What a silly error. Thanks.
$endgroup$
– user503154
Dec 20 '18 at 18:37
1
$begingroup$
Pretty slick. As a corollary (or a lemma toward), no square is of the form $4k+3$. Rather cute.
$endgroup$
– fleablood
Dec 20 '18 at 19:29
add a comment |
7
$begingroup$
"Suppose, for contradiction that $sqrt{3}$ is rational."
$endgroup$
– John
Dec 20 '18 at 18:34
2
$begingroup$
What a silly error. Thanks.
$endgroup$
– user503154
Dec 20 '18 at 18:37
1
$begingroup$
Pretty slick. As a corollary (or a lemma toward), no square is of the form $4k+3$. Rather cute.
$endgroup$
– fleablood
Dec 20 '18 at 19:29
7
7
$begingroup$
"Suppose, for contradiction that $sqrt{3}$ is rational."
$endgroup$
– John
Dec 20 '18 at 18:34
$begingroup$
"Suppose, for contradiction that $sqrt{3}$ is rational."
$endgroup$
– John
Dec 20 '18 at 18:34
2
2
$begingroup$
What a silly error. Thanks.
$endgroup$
– user503154
Dec 20 '18 at 18:37
$begingroup$
What a silly error. Thanks.
$endgroup$
– user503154
Dec 20 '18 at 18:37
1
1
$begingroup$
Pretty slick. As a corollary (or a lemma toward), no square is of the form $4k+3$. Rather cute.
$endgroup$
– fleablood
Dec 20 '18 at 19:29
$begingroup$
Pretty slick. As a corollary (or a lemma toward), no square is of the form $4k+3$. Rather cute.
$endgroup$
– fleablood
Dec 20 '18 at 19:29
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Your proof looks correct to me. Instead of doing the algebra at the end, you could reduce the equation $a^2 = 3b^2$ modulo $4$. If $a$ and $b$ are both odd, then $$a^2 equiv 1 mod 4,$$
and
begin{align*}
b^2 &equiv 1mod 4\
3b^2 &equiv 3mod 4,
end{align*}
contradiction.
Another approach is to note that in the prime factorization of $a^2 = 3b^2$, the power of $3$ dividing the left hand side is even, while the power of $3$ dividing the right hand side is odd.
$endgroup$
2
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
add a comment |
$begingroup$
Other proof
we know that
$$1<sqrt{3}<2$$
assume $a=bsqrt{3}in Bbb N$
with $b>1$.
Let $$A={cin Bbb N, c>1 : csqrt{3}in Bbb N}$$
$$Aneq emptyset $$
let $m=min A$.
then
$$alpha=m(sqrt{3}-1)in A text{ and } alpha<min A$$
which is a contradiction, thus $sqrt{3}notin Bbb Q$.
$endgroup$
1
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
1
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Your proof looks correct to me. Instead of doing the algebra at the end, you could reduce the equation $a^2 = 3b^2$ modulo $4$. If $a$ and $b$ are both odd, then $$a^2 equiv 1 mod 4,$$
and
begin{align*}
b^2 &equiv 1mod 4\
3b^2 &equiv 3mod 4,
end{align*}
contradiction.
Another approach is to note that in the prime factorization of $a^2 = 3b^2$, the power of $3$ dividing the left hand side is even, while the power of $3$ dividing the right hand side is odd.
$endgroup$
2
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
add a comment |
$begingroup$
Your proof looks correct to me. Instead of doing the algebra at the end, you could reduce the equation $a^2 = 3b^2$ modulo $4$. If $a$ and $b$ are both odd, then $$a^2 equiv 1 mod 4,$$
and
begin{align*}
b^2 &equiv 1mod 4\
3b^2 &equiv 3mod 4,
end{align*}
contradiction.
Another approach is to note that in the prime factorization of $a^2 = 3b^2$, the power of $3$ dividing the left hand side is even, while the power of $3$ dividing the right hand side is odd.
$endgroup$
2
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
add a comment |
$begingroup$
Your proof looks correct to me. Instead of doing the algebra at the end, you could reduce the equation $a^2 = 3b^2$ modulo $4$. If $a$ and $b$ are both odd, then $$a^2 equiv 1 mod 4,$$
and
begin{align*}
b^2 &equiv 1mod 4\
3b^2 &equiv 3mod 4,
end{align*}
contradiction.
Another approach is to note that in the prime factorization of $a^2 = 3b^2$, the power of $3$ dividing the left hand side is even, while the power of $3$ dividing the right hand side is odd.
$endgroup$
Your proof looks correct to me. Instead of doing the algebra at the end, you could reduce the equation $a^2 = 3b^2$ modulo $4$. If $a$ and $b$ are both odd, then $$a^2 equiv 1 mod 4,$$
and
begin{align*}
b^2 &equiv 1mod 4\
3b^2 &equiv 3mod 4,
end{align*}
contradiction.
Another approach is to note that in the prime factorization of $a^2 = 3b^2$, the power of $3$ dividing the left hand side is even, while the power of $3$ dividing the right hand side is odd.
answered Dec 20 '18 at 18:39
Alex KruckmanAlex Kruckman
28.2k32658
28.2k32658
2
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
add a comment |
2
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
2
2
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
$begingroup$
Thank you. That is a better method.
$endgroup$
– user503154
Dec 20 '18 at 18:53
add a comment |
$begingroup$
Other proof
we know that
$$1<sqrt{3}<2$$
assume $a=bsqrt{3}in Bbb N$
with $b>1$.
Let $$A={cin Bbb N, c>1 : csqrt{3}in Bbb N}$$
$$Aneq emptyset $$
let $m=min A$.
then
$$alpha=m(sqrt{3}-1)in A text{ and } alpha<min A$$
which is a contradiction, thus $sqrt{3}notin Bbb Q$.
$endgroup$
1
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
1
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
add a comment |
$begingroup$
Other proof
we know that
$$1<sqrt{3}<2$$
assume $a=bsqrt{3}in Bbb N$
with $b>1$.
Let $$A={cin Bbb N, c>1 : csqrt{3}in Bbb N}$$
$$Aneq emptyset $$
let $m=min A$.
then
$$alpha=m(sqrt{3}-1)in A text{ and } alpha<min A$$
which is a contradiction, thus $sqrt{3}notin Bbb Q$.
$endgroup$
1
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
1
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
add a comment |
$begingroup$
Other proof
we know that
$$1<sqrt{3}<2$$
assume $a=bsqrt{3}in Bbb N$
with $b>1$.
Let $$A={cin Bbb N, c>1 : csqrt{3}in Bbb N}$$
$$Aneq emptyset $$
let $m=min A$.
then
$$alpha=m(sqrt{3}-1)in A text{ and } alpha<min A$$
which is a contradiction, thus $sqrt{3}notin Bbb Q$.
$endgroup$
Other proof
we know that
$$1<sqrt{3}<2$$
assume $a=bsqrt{3}in Bbb N$
with $b>1$.
Let $$A={cin Bbb N, c>1 : csqrt{3}in Bbb N}$$
$$Aneq emptyset $$
let $m=min A$.
then
$$alpha=m(sqrt{3}-1)in A text{ and } alpha<min A$$
which is a contradiction, thus $sqrt{3}notin Bbb Q$.
edited Dec 20 '18 at 19:09
answered Dec 20 '18 at 19:00
hamam_Abdallahhamam_Abdallah
38.2k21634
38.2k21634
1
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
1
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
add a comment |
1
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
1
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
1
1
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
$begingroup$
More conceptually: the denominator set $A$ contains coprimes $b$ and $a$ (by $a^2/b^2=3,Rightarrow,,a/b = 3b/a),$ thus since $A$ is closed under subtractiion it is closed under gcd, so it contains $1 = gcd(a,b),,$ i.e. $,sqrt 3,$ can be written with denominator $1$, i.e. $,sqrt 3in Bbb Z,,$ contradiction. See my posts on denominator descent for more on this viewpoint.
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:12
1
1
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
$begingroup$
Alternatively by $,a^2/b = 3b,Rightarrow, bmid a^2,$ so $,b=1,,$ by $,a,b,$ coprime and Euclid's lemma (generally. the least denominator of a fraction divides every denominator, a special case of the fact that ideals are principal in $,Bbb Z) $
$endgroup$
– Bill Dubuque
Dec 20 '18 at 19:23
add a comment |
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7
$begingroup$
"Suppose, for contradiction that $sqrt{3}$ is rational."
$endgroup$
– John
Dec 20 '18 at 18:34
2
$begingroup$
What a silly error. Thanks.
$endgroup$
– user503154
Dec 20 '18 at 18:37
1
$begingroup$
Pretty slick. As a corollary (or a lemma toward), no square is of the form $4k+3$. Rather cute.
$endgroup$
– fleablood
Dec 20 '18 at 19:29