Prove the following: Let $n$ be an integer. If $2|n^2$, then $4|n^2$.
$begingroup$
I came up with the following:
$2|n^2$ implies that $2|n*n$. We proved in class that if $q|b*p$, then $q|b$ or $q|p$. Therefore, if $2|n^2$, then $2|n$ or $2|n$.
So, $2|n$ implies $n=2k$, for some $k ∈ Z$.
So, $n^2|(2k)^2$,
$n^2 = 4k^2$, where $k^2 ∈ Z$.
So, if $2|n^2$, then $4|n^2$ as desired.
What do you all think?
proof-writing
$endgroup$
|
show 1 more comment
$begingroup$
I came up with the following:
$2|n^2$ implies that $2|n*n$. We proved in class that if $q|b*p$, then $q|b$ or $q|p$. Therefore, if $2|n^2$, then $2|n$ or $2|n$.
So, $2|n$ implies $n=2k$, for some $k ∈ Z$.
So, $n^2|(2k)^2$,
$n^2 = 4k^2$, where $k^2 ∈ Z$.
So, if $2|n^2$, then $4|n^2$ as desired.
What do you all think?
proof-writing
$endgroup$
$begingroup$
Well if $n$ is odd, then $2nmid n^2$ so you're done with that case.
$endgroup$
– saulspatz
Dec 6 '18 at 4:24
$begingroup$
Should it be "if $2mid color{red}{n}$ then $4mid n^2$"?
$endgroup$
– Dave
Dec 6 '18 at 4:24
$begingroup$
See the statement is - if $2|n^2$ then $4|n^2$. You don't need to think about odd $n$.
$endgroup$
– tarit goswami
Dec 6 '18 at 4:31
$begingroup$
Hi and welcome to MSE. Have you done prime factorisation? Assuming so, what are the properties of the exponents of the prime factors in the every factorisation of a square number?
$endgroup$
– user334732
Dec 6 '18 at 18:57
$begingroup$
Try adding the answer to this to your question: Is this a problem from a text, or from a class? If from a class, which one, and what theorems have they seen recently? If a text, which one? For example, are they in a context of abstract algebra, or number theory, or intro to proofs, or something else? See this discussion: chat.stackexchange.com/transcript/message/47930322#47930322
$endgroup$
– user334732
Dec 6 '18 at 19:46
|
show 1 more comment
$begingroup$
I came up with the following:
$2|n^2$ implies that $2|n*n$. We proved in class that if $q|b*p$, then $q|b$ or $q|p$. Therefore, if $2|n^2$, then $2|n$ or $2|n$.
So, $2|n$ implies $n=2k$, for some $k ∈ Z$.
So, $n^2|(2k)^2$,
$n^2 = 4k^2$, where $k^2 ∈ Z$.
So, if $2|n^2$, then $4|n^2$ as desired.
What do you all think?
proof-writing
$endgroup$
I came up with the following:
$2|n^2$ implies that $2|n*n$. We proved in class that if $q|b*p$, then $q|b$ or $q|p$. Therefore, if $2|n^2$, then $2|n$ or $2|n$.
So, $2|n$ implies $n=2k$, for some $k ∈ Z$.
So, $n^2|(2k)^2$,
$n^2 = 4k^2$, where $k^2 ∈ Z$.
So, if $2|n^2$, then $4|n^2$ as desired.
What do you all think?
proof-writing
proof-writing
edited Dec 8 '18 at 1:22
Mettal
asked Dec 6 '18 at 4:21
MettalMettal
257
257
$begingroup$
Well if $n$ is odd, then $2nmid n^2$ so you're done with that case.
$endgroup$
– saulspatz
Dec 6 '18 at 4:24
$begingroup$
Should it be "if $2mid color{red}{n}$ then $4mid n^2$"?
$endgroup$
– Dave
Dec 6 '18 at 4:24
$begingroup$
See the statement is - if $2|n^2$ then $4|n^2$. You don't need to think about odd $n$.
$endgroup$
– tarit goswami
Dec 6 '18 at 4:31
$begingroup$
Hi and welcome to MSE. Have you done prime factorisation? Assuming so, what are the properties of the exponents of the prime factors in the every factorisation of a square number?
$endgroup$
– user334732
Dec 6 '18 at 18:57
$begingroup$
Try adding the answer to this to your question: Is this a problem from a text, or from a class? If from a class, which one, and what theorems have they seen recently? If a text, which one? For example, are they in a context of abstract algebra, or number theory, or intro to proofs, or something else? See this discussion: chat.stackexchange.com/transcript/message/47930322#47930322
$endgroup$
– user334732
Dec 6 '18 at 19:46
|
show 1 more comment
$begingroup$
Well if $n$ is odd, then $2nmid n^2$ so you're done with that case.
$endgroup$
– saulspatz
Dec 6 '18 at 4:24
$begingroup$
Should it be "if $2mid color{red}{n}$ then $4mid n^2$"?
$endgroup$
– Dave
Dec 6 '18 at 4:24
$begingroup$
See the statement is - if $2|n^2$ then $4|n^2$. You don't need to think about odd $n$.
$endgroup$
– tarit goswami
Dec 6 '18 at 4:31
$begingroup$
Hi and welcome to MSE. Have you done prime factorisation? Assuming so, what are the properties of the exponents of the prime factors in the every factorisation of a square number?
$endgroup$
– user334732
Dec 6 '18 at 18:57
$begingroup$
Try adding the answer to this to your question: Is this a problem from a text, or from a class? If from a class, which one, and what theorems have they seen recently? If a text, which one? For example, are they in a context of abstract algebra, or number theory, or intro to proofs, or something else? See this discussion: chat.stackexchange.com/transcript/message/47930322#47930322
$endgroup$
– user334732
Dec 6 '18 at 19:46
$begingroup$
Well if $n$ is odd, then $2nmid n^2$ so you're done with that case.
$endgroup$
– saulspatz
Dec 6 '18 at 4:24
$begingroup$
Well if $n$ is odd, then $2nmid n^2$ so you're done with that case.
$endgroup$
– saulspatz
Dec 6 '18 at 4:24
$begingroup$
Should it be "if $2mid color{red}{n}$ then $4mid n^2$"?
$endgroup$
– Dave
Dec 6 '18 at 4:24
$begingroup$
Should it be "if $2mid color{red}{n}$ then $4mid n^2$"?
$endgroup$
– Dave
Dec 6 '18 at 4:24
$begingroup$
See the statement is - if $2|n^2$ then $4|n^2$. You don't need to think about odd $n$.
$endgroup$
– tarit goswami
Dec 6 '18 at 4:31
$begingroup$
See the statement is - if $2|n^2$ then $4|n^2$. You don't need to think about odd $n$.
$endgroup$
– tarit goswami
Dec 6 '18 at 4:31
$begingroup$
Hi and welcome to MSE. Have you done prime factorisation? Assuming so, what are the properties of the exponents of the prime factors in the every factorisation of a square number?
$endgroup$
– user334732
Dec 6 '18 at 18:57
$begingroup$
Hi and welcome to MSE. Have you done prime factorisation? Assuming so, what are the properties of the exponents of the prime factors in the every factorisation of a square number?
$endgroup$
– user334732
Dec 6 '18 at 18:57
$begingroup$
Try adding the answer to this to your question: Is this a problem from a text, or from a class? If from a class, which one, and what theorems have they seen recently? If a text, which one? For example, are they in a context of abstract algebra, or number theory, or intro to proofs, or something else? See this discussion: chat.stackexchange.com/transcript/message/47930322#47930322
$endgroup$
– user334732
Dec 6 '18 at 19:46
$begingroup$
Try adding the answer to this to your question: Is this a problem from a text, or from a class? If from a class, which one, and what theorems have they seen recently? If a text, which one? For example, are they in a context of abstract algebra, or number theory, or intro to proofs, or something else? See this discussion: chat.stackexchange.com/transcript/message/47930322#47930322
$endgroup$
– user334732
Dec 6 '18 at 19:46
|
show 1 more comment
4 Answers
4
active
oldest
votes
$begingroup$
Suppose not. Then $n^2=2times (2m+1)$ for some positive integer $m$. But that means, $n^2=4m+2$. Which is a contradiction, as, any perfect square can leave remainder $0,1$ after division by $4$.
Here is a more satisfactory arument. A statement and it's contrapositive statement has same truth value. Here, the contrapositive statement of $2|n^2implies 4|n^2$ is $4nmid n^2implies 2nmid n$. So, if we can prove the contrapositive statement is true then we are done.
$$4nmid n^2implies 2nmid nimplies 2nmid ncdot n $$
So we are sone!
$endgroup$
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
add a comment |
$begingroup$
If $2|n^2$, $n^2$ is even. Note that $n^2$ is even if and only if $n$ is even (in $Bbb Z$). So $n=2m $ for some $m in Bbb Z$. Hence $n^2=4m^2$.
$endgroup$
add a comment |
$begingroup$
If $2|m^2$ then it means its square root $m$ must be even, or else $m$ would be irrational. If it were odd then there would be no $2$ as part of its prime factors, so multiplying it by itself would not create an even number. We can define $n$ such that $2n=m$. Since $m^2=(2n)^2=4n^2$, then we can easily show $4|4n^2$ and therefore $4|m^2$.
$endgroup$
add a comment |
$begingroup$
An import ant part of writing any proof is to determine from the outset what you can assume to be true.
If you can assume the fundamental theorem of arithmetic then you have the unique prime factorisation of $n=p_1^{i_2}cdot p_2^{i_2}cdot p_3^{i_3}ldots$
Then the prime factorisation of $n^2=p_1^{2i_2}cdot p_2^{2i_2}cdot p_3^{2i_3}ldots$
Then since every exponent is even, the power of every prime factor of $n^2$ is either $1$ or a power of $p^2$, which generalises the result you need to every prime.
(In $2^3=8$, 2 is the prime, 3 is the exponent and 8 is the power).
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Suppose not. Then $n^2=2times (2m+1)$ for some positive integer $m$. But that means, $n^2=4m+2$. Which is a contradiction, as, any perfect square can leave remainder $0,1$ after division by $4$.
Here is a more satisfactory arument. A statement and it's contrapositive statement has same truth value. Here, the contrapositive statement of $2|n^2implies 4|n^2$ is $4nmid n^2implies 2nmid n$. So, if we can prove the contrapositive statement is true then we are done.
$$4nmid n^2implies 2nmid nimplies 2nmid ncdot n $$
So we are sone!
$endgroup$
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
add a comment |
$begingroup$
Suppose not. Then $n^2=2times (2m+1)$ for some positive integer $m$. But that means, $n^2=4m+2$. Which is a contradiction, as, any perfect square can leave remainder $0,1$ after division by $4$.
Here is a more satisfactory arument. A statement and it's contrapositive statement has same truth value. Here, the contrapositive statement of $2|n^2implies 4|n^2$ is $4nmid n^2implies 2nmid n$. So, if we can prove the contrapositive statement is true then we are done.
$$4nmid n^2implies 2nmid nimplies 2nmid ncdot n $$
So we are sone!
$endgroup$
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
add a comment |
$begingroup$
Suppose not. Then $n^2=2times (2m+1)$ for some positive integer $m$. But that means, $n^2=4m+2$. Which is a contradiction, as, any perfect square can leave remainder $0,1$ after division by $4$.
Here is a more satisfactory arument. A statement and it's contrapositive statement has same truth value. Here, the contrapositive statement of $2|n^2implies 4|n^2$ is $4nmid n^2implies 2nmid n$. So, if we can prove the contrapositive statement is true then we are done.
$$4nmid n^2implies 2nmid nimplies 2nmid ncdot n $$
So we are sone!
$endgroup$
Suppose not. Then $n^2=2times (2m+1)$ for some positive integer $m$. But that means, $n^2=4m+2$. Which is a contradiction, as, any perfect square can leave remainder $0,1$ after division by $4$.
Here is a more satisfactory arument. A statement and it's contrapositive statement has same truth value. Here, the contrapositive statement of $2|n^2implies 4|n^2$ is $4nmid n^2implies 2nmid n$. So, if we can prove the contrapositive statement is true then we are done.
$$4nmid n^2implies 2nmid nimplies 2nmid ncdot n $$
So we are sone!
edited Dec 6 '18 at 4:48
answered Dec 6 '18 at 4:35
tarit goswamitarit goswami
1,7591421
1,7591421
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
add a comment |
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
Is there any problem to understand this?
$endgroup$
– tarit goswami
Dec 6 '18 at 4:56
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
$begingroup$
No. I think I got it. Thank you!
$endgroup$
– Mettal
Dec 6 '18 at 4:58
add a comment |
$begingroup$
If $2|n^2$, $n^2$ is even. Note that $n^2$ is even if and only if $n$ is even (in $Bbb Z$). So $n=2m $ for some $m in Bbb Z$. Hence $n^2=4m^2$.
$endgroup$
add a comment |
$begingroup$
If $2|n^2$, $n^2$ is even. Note that $n^2$ is even if and only if $n$ is even (in $Bbb Z$). So $n=2m $ for some $m in Bbb Z$. Hence $n^2=4m^2$.
$endgroup$
add a comment |
$begingroup$
If $2|n^2$, $n^2$ is even. Note that $n^2$ is even if and only if $n$ is even (in $Bbb Z$). So $n=2m $ for some $m in Bbb Z$. Hence $n^2=4m^2$.
$endgroup$
If $2|n^2$, $n^2$ is even. Note that $n^2$ is even if and only if $n$ is even (in $Bbb Z$). So $n=2m $ for some $m in Bbb Z$. Hence $n^2=4m^2$.
answered Dec 6 '18 at 4:28
Thomas ShelbyThomas Shelby
2,765421
2,765421
add a comment |
add a comment |
$begingroup$
If $2|m^2$ then it means its square root $m$ must be even, or else $m$ would be irrational. If it were odd then there would be no $2$ as part of its prime factors, so multiplying it by itself would not create an even number. We can define $n$ such that $2n=m$. Since $m^2=(2n)^2=4n^2$, then we can easily show $4|4n^2$ and therefore $4|m^2$.
$endgroup$
add a comment |
$begingroup$
If $2|m^2$ then it means its square root $m$ must be even, or else $m$ would be irrational. If it were odd then there would be no $2$ as part of its prime factors, so multiplying it by itself would not create an even number. We can define $n$ such that $2n=m$. Since $m^2=(2n)^2=4n^2$, then we can easily show $4|4n^2$ and therefore $4|m^2$.
$endgroup$
add a comment |
$begingroup$
If $2|m^2$ then it means its square root $m$ must be even, or else $m$ would be irrational. If it were odd then there would be no $2$ as part of its prime factors, so multiplying it by itself would not create an even number. We can define $n$ such that $2n=m$. Since $m^2=(2n)^2=4n^2$, then we can easily show $4|4n^2$ and therefore $4|m^2$.
$endgroup$
If $2|m^2$ then it means its square root $m$ must be even, or else $m$ would be irrational. If it were odd then there would be no $2$ as part of its prime factors, so multiplying it by itself would not create an even number. We can define $n$ such that $2n=m$. Since $m^2=(2n)^2=4n^2$, then we can easily show $4|4n^2$ and therefore $4|m^2$.
edited Dec 6 '18 at 4:45
answered Dec 6 '18 at 4:38
KykyKyky
462213
462213
add a comment |
add a comment |
$begingroup$
An import ant part of writing any proof is to determine from the outset what you can assume to be true.
If you can assume the fundamental theorem of arithmetic then you have the unique prime factorisation of $n=p_1^{i_2}cdot p_2^{i_2}cdot p_3^{i_3}ldots$
Then the prime factorisation of $n^2=p_1^{2i_2}cdot p_2^{2i_2}cdot p_3^{2i_3}ldots$
Then since every exponent is even, the power of every prime factor of $n^2$ is either $1$ or a power of $p^2$, which generalises the result you need to every prime.
(In $2^3=8$, 2 is the prime, 3 is the exponent and 8 is the power).
$endgroup$
add a comment |
$begingroup$
An import ant part of writing any proof is to determine from the outset what you can assume to be true.
If you can assume the fundamental theorem of arithmetic then you have the unique prime factorisation of $n=p_1^{i_2}cdot p_2^{i_2}cdot p_3^{i_3}ldots$
Then the prime factorisation of $n^2=p_1^{2i_2}cdot p_2^{2i_2}cdot p_3^{2i_3}ldots$
Then since every exponent is even, the power of every prime factor of $n^2$ is either $1$ or a power of $p^2$, which generalises the result you need to every prime.
(In $2^3=8$, 2 is the prime, 3 is the exponent and 8 is the power).
$endgroup$
add a comment |
$begingroup$
An import ant part of writing any proof is to determine from the outset what you can assume to be true.
If you can assume the fundamental theorem of arithmetic then you have the unique prime factorisation of $n=p_1^{i_2}cdot p_2^{i_2}cdot p_3^{i_3}ldots$
Then the prime factorisation of $n^2=p_1^{2i_2}cdot p_2^{2i_2}cdot p_3^{2i_3}ldots$
Then since every exponent is even, the power of every prime factor of $n^2$ is either $1$ or a power of $p^2$, which generalises the result you need to every prime.
(In $2^3=8$, 2 is the prime, 3 is the exponent and 8 is the power).
$endgroup$
An import ant part of writing any proof is to determine from the outset what you can assume to be true.
If you can assume the fundamental theorem of arithmetic then you have the unique prime factorisation of $n=p_1^{i_2}cdot p_2^{i_2}cdot p_3^{i_3}ldots$
Then the prime factorisation of $n^2=p_1^{2i_2}cdot p_2^{2i_2}cdot p_3^{2i_3}ldots$
Then since every exponent is even, the power of every prime factor of $n^2$ is either $1$ or a power of $p^2$, which generalises the result you need to every prime.
(In $2^3=8$, 2 is the prime, 3 is the exponent and 8 is the power).
answered Dec 7 '18 at 10:37
user334732user334732
4,25911240
4,25911240
add a comment |
add a comment |
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$begingroup$
Well if $n$ is odd, then $2nmid n^2$ so you're done with that case.
$endgroup$
– saulspatz
Dec 6 '18 at 4:24
$begingroup$
Should it be "if $2mid color{red}{n}$ then $4mid n^2$"?
$endgroup$
– Dave
Dec 6 '18 at 4:24
$begingroup$
See the statement is - if $2|n^2$ then $4|n^2$. You don't need to think about odd $n$.
$endgroup$
– tarit goswami
Dec 6 '18 at 4:31
$begingroup$
Hi and welcome to MSE. Have you done prime factorisation? Assuming so, what are the properties of the exponents of the prime factors in the every factorisation of a square number?
$endgroup$
– user334732
Dec 6 '18 at 18:57
$begingroup$
Try adding the answer to this to your question: Is this a problem from a text, or from a class? If from a class, which one, and what theorems have they seen recently? If a text, which one? For example, are they in a context of abstract algebra, or number theory, or intro to proofs, or something else? See this discussion: chat.stackexchange.com/transcript/message/47930322#47930322
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– user334732
Dec 6 '18 at 19:46