Prove that $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number
Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.
I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?
discrete-mathematics modular-arithmetic divisibility
add a comment |
Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.
I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?
discrete-mathematics modular-arithmetic divisibility
2
It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27
yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31
It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32
You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32
See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09
add a comment |
Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.
I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?
discrete-mathematics modular-arithmetic divisibility
Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.
I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?
discrete-mathematics modular-arithmetic divisibility
discrete-mathematics modular-arithmetic divisibility
asked Nov 22 at 21:24
Marko Škorić
70310
70310
2
It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27
yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31
It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32
You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32
See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09
add a comment |
2
It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27
yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31
It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32
You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32
See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09
2
2
It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27
It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27
yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31
yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31
It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32
It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32
You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32
You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32
See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09
See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09
add a comment |
4 Answers
4
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oldest
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By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$
Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's
Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$
But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$
add a comment |
It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
add a comment |
$m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.
add a comment |
Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
$$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.
add a comment |
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4 Answers
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4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$
Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's
Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$
But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$
add a comment |
By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$
Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's
Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$
But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$
add a comment |
By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$
Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's
Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$
But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$
By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$
Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's
Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$
But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$
edited Nov 22 at 23:21
answered Nov 22 at 22:17
Bill Dubuque
208k29190626
208k29190626
add a comment |
add a comment |
It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
add a comment |
It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
add a comment |
It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$
It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$
answered Nov 22 at 21:33
Mostafa Ayaz
13.7k3836
13.7k3836
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
add a comment |
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
yep, I suspect the same
– Masacroso
Nov 22 at 21:35
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
Just requires some manipulation in numerator or denominators........
– Mostafa Ayaz
Nov 22 at 21:36
add a comment |
$m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.
add a comment |
$m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.
add a comment |
$m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.
$m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.
answered Nov 22 at 21:32
John_Wick
1,356111
1,356111
add a comment |
add a comment |
Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
$$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.
add a comment |
Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
$$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.
add a comment |
Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
$$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.
Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
$$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.
answered Nov 22 at 22:32
Jack D'Aurizio
286k33279655
286k33279655
add a comment |
add a comment |
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2
It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27
yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31
It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32
You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32
See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09