Can any improvements be made to my proof that “$sqrt{3} $ is irrational”?












5












$begingroup$


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.










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$endgroup$








  • 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
















5












$begingroup$


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.










share|cite|improve this question











$endgroup$








  • 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














5












5








5


0



$begingroup$


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.










share|cite|improve this question











$endgroup$




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






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share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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














  • 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










2 Answers
2






active

oldest

votes


















3












$begingroup$

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.






share|cite|improve this answer









$endgroup$









  • 2




    $begingroup$
    Thank you. That is a better method.
    $endgroup$
    – user503154
    Dec 20 '18 at 18:53



















0












$begingroup$

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 $sqrt{3}notin Bbb Q$.






share|cite|improve this answer











$endgroup$









  • 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













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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

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.






share|cite|improve this answer









$endgroup$









  • 2




    $begingroup$
    Thank you. That is a better method.
    $endgroup$
    – user503154
    Dec 20 '18 at 18:53
















3












$begingroup$

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.






share|cite|improve this answer









$endgroup$









  • 2




    $begingroup$
    Thank you. That is a better method.
    $endgroup$
    – user503154
    Dec 20 '18 at 18:53














3












3








3





$begingroup$

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.






share|cite|improve this answer









$endgroup$



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.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










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














  • 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











0












$begingroup$

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 $sqrt{3}notin Bbb Q$.






share|cite|improve this answer











$endgroup$









  • 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


















0












$begingroup$

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 $sqrt{3}notin Bbb Q$.






share|cite|improve this answer











$endgroup$









  • 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
















0












0








0





$begingroup$

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 $sqrt{3}notin Bbb Q$.






share|cite|improve this answer











$endgroup$



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 $sqrt{3}notin Bbb Q$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








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
















  • 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




















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