find all $k$ such that $k^5+3$ is divisible by $k^2+1$
$begingroup$
That's it. I have a solution with substitutions, but it's tedious and not too generic. Is there a solution using some cool theorem for polynomials divisibility?
polynomials diophantine-equations
$endgroup$
add a comment |
$begingroup$
That's it. I have a solution with substitutions, but it's tedious and not too generic. Is there a solution using some cool theorem for polynomials divisibility?
polynomials diophantine-equations
$endgroup$
add a comment |
$begingroup$
That's it. I have a solution with substitutions, but it's tedious and not too generic. Is there a solution using some cool theorem for polynomials divisibility?
polynomials diophantine-equations
$endgroup$
That's it. I have a solution with substitutions, but it's tedious and not too generic. Is there a solution using some cool theorem for polynomials divisibility?
polynomials diophantine-equations
polynomials diophantine-equations
asked Nov 29 '18 at 15:40
kirilloidkirilloid
1656
1656
add a comment |
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
From $k^5+3=(k^2+1)(k^3-k)+k+3$ we see that $k^2+1mid k^5+3$ implies $k^2+1mid k+3$. If $k+3not=0$, we must have $k^2+1le|k+3|le|k|+3$, which can be rewritten as $|k|^2le|k|+2$. It's not hard to see that this implies $|k|le2$, at which point one can simply check the five possible values of $k$ (remembering to check $k=-3$ as well).
Remark (added later): This approach extends to any pair of polynomials in $k$ (with integer coefficients): If the polynomial division of $Q(k)$ by $P(k)$ leaves a nonzero remainder $R(k)$ of degree less than the degree of $P$, then the set of $k$ for which $P(k)mid Q(k)$ is necessarily finite, limited to the integer roots of $R(k)$ and $k$'s satisfying the inequality $|P(k)|le|R(k)|$. In any given case there may be other (e.g., congruence) considerations that limit the search more efficiently; for example, $P(k)=k^2+k+2$ is always even while $Q(k)=k^5+k^3+1$ is always odd, so we don't have to search at all. It'd be nice to have an example where there are some solutions to search for that are cumbersome for this approach to find but are nonetheless easy to pinpoint by another approach.
$endgroup$
add a comment |
$begingroup$
Since $$k^2+1mid (k^2+1)(k^2-1)k = k^5-k$$ we have $$k^2+1mid (k^5+3)-(k^5-k) = k+3$$
Now $$k^2+1mid (k+3)(k-3)=k^2-9$$
So $$k^2+1mid (k^2+1)-(k^2-9) =10$$
So $k^2+1in{1,2,5,10}$ so $kin{0,pm1,pm2,pm 3}$.
$endgroup$
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
add a comment |
$begingroup$
$!bmod k^2!+!1!:, color{#c00}{k^2equiv-1},Rightarrow, k^4equiv 1, $ so $, color{#0a0}0equiv k(k^4)!+!3equiv color{#0a0}{k!+!3}$
therefore $ color{#0a0}{0equiv (3!+!k)}(3!-!k)equiv 9-color{#c00}{k^2}equiv 10, $ so $, bbox[5px,border:1px solid red]{k^2!+!1mid 10}$
Remark $ color{#c00}{k^2equiv -1},$ means $,k,$ behaves like $,i,$ so the above is essentially
$qquadqquadqquadquadbegin{align} &0equiv z equiv 3!+!i^5 equiv 3!+!i\[.4em]
Rightarrow &0 equiv zbar z equiv (3!+!i)(3!-!i) equiv 10end{align}$
which has a natural interpretation in Gaussian integer arithmetic (or complex numbers).
As for a "solution using polynomials", the above computation is generic and it translates to the following Euclidean gcd (or ideal) calculation with polynomials
$qquadqquad(x^2!+!1,!overbrace{x^5!+!3!!!}^{largecolor{#c00}{x^2 equiv -1}}), =, (!overbrace{x^2!+!1}^{largecolor{#0a0}{x equiv -3}},,x!+!3), = ,(10,,x!+!3)$
using Euclid's $, (a,,b) = (a,, bbmod a)., $ We could go further and extract the Bezout relation giving $10$ as a linear combination of $,x^2!+!1,,x^5!+!3,,$ but there is no need to do so for the problem at hand.
$endgroup$
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
1
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
1
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
|
show 1 more comment
$begingroup$
Solution with substitutions.
If $k^5+3$ is divisible by $k^2+1$, it could be expressed as $(k^2+1)(k^3+n)$, which if expanded, leads us to $k^3+nk^2+n-3$.
Now let's make another substitution by introducing another variable $a: k=-(n+a)$, which will get us $-an^2+(1-2a^2)n-(a^3+3)$. It is solvable for $n$ only when $Dge0$ or $-4a^2-12a+1ge0$.
Roots of the last [expression turned into an] equation are $-3pmfrac{sqrt{10}}{2}$, and since we are looking only for integers, we need to check $ain{-3,-2,-1,0}$.
Now we can start unrolling our substitutions. For all valid values of $a$, solve $-an^2+(1-2a^2)n-(a^3+3)$ for $n$, and get values of $k$ directly as $-(n+a)$
$$begin {array} {r|r} equation & a & n & k\
hline
3n^2-17n+24=0 & -3 & 3 & 0 \
3n^2-17n+24=0 & -3 & frac83 & - \
2n^2-7n+5=0 & -2 & 1 & 1 \
2n^2-7n+5=0 & -2 & 2.5 & - \
n^2-n-2=0 & -1 & -1 & 2 \
n^2-n-2=0 & -1 & 2 & -1 \
n-3=0 & 0 & 3 & -3 \
end {array}$$
$endgroup$
add a comment |
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4 Answers
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4 Answers
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$begingroup$
From $k^5+3=(k^2+1)(k^3-k)+k+3$ we see that $k^2+1mid k^5+3$ implies $k^2+1mid k+3$. If $k+3not=0$, we must have $k^2+1le|k+3|le|k|+3$, which can be rewritten as $|k|^2le|k|+2$. It's not hard to see that this implies $|k|le2$, at which point one can simply check the five possible values of $k$ (remembering to check $k=-3$ as well).
Remark (added later): This approach extends to any pair of polynomials in $k$ (with integer coefficients): If the polynomial division of $Q(k)$ by $P(k)$ leaves a nonzero remainder $R(k)$ of degree less than the degree of $P$, then the set of $k$ for which $P(k)mid Q(k)$ is necessarily finite, limited to the integer roots of $R(k)$ and $k$'s satisfying the inequality $|P(k)|le|R(k)|$. In any given case there may be other (e.g., congruence) considerations that limit the search more efficiently; for example, $P(k)=k^2+k+2$ is always even while $Q(k)=k^5+k^3+1$ is always odd, so we don't have to search at all. It'd be nice to have an example where there are some solutions to search for that are cumbersome for this approach to find but are nonetheless easy to pinpoint by another approach.
$endgroup$
add a comment |
$begingroup$
From $k^5+3=(k^2+1)(k^3-k)+k+3$ we see that $k^2+1mid k^5+3$ implies $k^2+1mid k+3$. If $k+3not=0$, we must have $k^2+1le|k+3|le|k|+3$, which can be rewritten as $|k|^2le|k|+2$. It's not hard to see that this implies $|k|le2$, at which point one can simply check the five possible values of $k$ (remembering to check $k=-3$ as well).
Remark (added later): This approach extends to any pair of polynomials in $k$ (with integer coefficients): If the polynomial division of $Q(k)$ by $P(k)$ leaves a nonzero remainder $R(k)$ of degree less than the degree of $P$, then the set of $k$ for which $P(k)mid Q(k)$ is necessarily finite, limited to the integer roots of $R(k)$ and $k$'s satisfying the inequality $|P(k)|le|R(k)|$. In any given case there may be other (e.g., congruence) considerations that limit the search more efficiently; for example, $P(k)=k^2+k+2$ is always even while $Q(k)=k^5+k^3+1$ is always odd, so we don't have to search at all. It'd be nice to have an example where there are some solutions to search for that are cumbersome for this approach to find but are nonetheless easy to pinpoint by another approach.
$endgroup$
add a comment |
$begingroup$
From $k^5+3=(k^2+1)(k^3-k)+k+3$ we see that $k^2+1mid k^5+3$ implies $k^2+1mid k+3$. If $k+3not=0$, we must have $k^2+1le|k+3|le|k|+3$, which can be rewritten as $|k|^2le|k|+2$. It's not hard to see that this implies $|k|le2$, at which point one can simply check the five possible values of $k$ (remembering to check $k=-3$ as well).
Remark (added later): This approach extends to any pair of polynomials in $k$ (with integer coefficients): If the polynomial division of $Q(k)$ by $P(k)$ leaves a nonzero remainder $R(k)$ of degree less than the degree of $P$, then the set of $k$ for which $P(k)mid Q(k)$ is necessarily finite, limited to the integer roots of $R(k)$ and $k$'s satisfying the inequality $|P(k)|le|R(k)|$. In any given case there may be other (e.g., congruence) considerations that limit the search more efficiently; for example, $P(k)=k^2+k+2$ is always even while $Q(k)=k^5+k^3+1$ is always odd, so we don't have to search at all. It'd be nice to have an example where there are some solutions to search for that are cumbersome for this approach to find but are nonetheless easy to pinpoint by another approach.
$endgroup$
From $k^5+3=(k^2+1)(k^3-k)+k+3$ we see that $k^2+1mid k^5+3$ implies $k^2+1mid k+3$. If $k+3not=0$, we must have $k^2+1le|k+3|le|k|+3$, which can be rewritten as $|k|^2le|k|+2$. It's not hard to see that this implies $|k|le2$, at which point one can simply check the five possible values of $k$ (remembering to check $k=-3$ as well).
Remark (added later): This approach extends to any pair of polynomials in $k$ (with integer coefficients): If the polynomial division of $Q(k)$ by $P(k)$ leaves a nonzero remainder $R(k)$ of degree less than the degree of $P$, then the set of $k$ for which $P(k)mid Q(k)$ is necessarily finite, limited to the integer roots of $R(k)$ and $k$'s satisfying the inequality $|P(k)|le|R(k)|$. In any given case there may be other (e.g., congruence) considerations that limit the search more efficiently; for example, $P(k)=k^2+k+2$ is always even while $Q(k)=k^5+k^3+1$ is always odd, so we don't have to search at all. It'd be nice to have an example where there are some solutions to search for that are cumbersome for this approach to find but are nonetheless easy to pinpoint by another approach.
edited Nov 29 '18 at 17:11
answered Nov 29 '18 at 15:52
Barry CipraBarry Cipra
59.4k653125
59.4k653125
add a comment |
add a comment |
$begingroup$
Since $$k^2+1mid (k^2+1)(k^2-1)k = k^5-k$$ we have $$k^2+1mid (k^5+3)-(k^5-k) = k+3$$
Now $$k^2+1mid (k+3)(k-3)=k^2-9$$
So $$k^2+1mid (k^2+1)-(k^2-9) =10$$
So $k^2+1in{1,2,5,10}$ so $kin{0,pm1,pm2,pm 3}$.
$endgroup$
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
add a comment |
$begingroup$
Since $$k^2+1mid (k^2+1)(k^2-1)k = k^5-k$$ we have $$k^2+1mid (k^5+3)-(k^5-k) = k+3$$
Now $$k^2+1mid (k+3)(k-3)=k^2-9$$
So $$k^2+1mid (k^2+1)-(k^2-9) =10$$
So $k^2+1in{1,2,5,10}$ so $kin{0,pm1,pm2,pm 3}$.
$endgroup$
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
add a comment |
$begingroup$
Since $$k^2+1mid (k^2+1)(k^2-1)k = k^5-k$$ we have $$k^2+1mid (k^5+3)-(k^5-k) = k+3$$
Now $$k^2+1mid (k+3)(k-3)=k^2-9$$
So $$k^2+1mid (k^2+1)-(k^2-9) =10$$
So $k^2+1in{1,2,5,10}$ so $kin{0,pm1,pm2,pm 3}$.
$endgroup$
Since $$k^2+1mid (k^2+1)(k^2-1)k = k^5-k$$ we have $$k^2+1mid (k^5+3)-(k^5-k) = k+3$$
Now $$k^2+1mid (k+3)(k-3)=k^2-9$$
So $$k^2+1mid (k^2+1)-(k^2-9) =10$$
So $k^2+1in{1,2,5,10}$ so $kin{0,pm1,pm2,pm 3}$.
answered Nov 29 '18 at 15:49
greedoidgreedoid
38.7k114797
38.7k114797
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
add a comment |
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
-2 and 3 are not solutions, but OK, checking only some of them is not much
$endgroup$
– kirilloid
Nov 29 '18 at 16:07
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
I'm not sure I understand the last transition. You added $k^2+1$ and got a tautology $10=10$, where does $kin{1,2,5,10}$ come from?
$endgroup$
– kirilloid
Nov 29 '18 at 16:20
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
$begingroup$
Nevermind, for get about tautology. How do we get $k^2+1in{1,2,5,10}$? $k^2+1 | 10 = 0$ so $10$ should be a multiple of $k^2+1$, let's check all divisors of $10$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:34
add a comment |
$begingroup$
$!bmod k^2!+!1!:, color{#c00}{k^2equiv-1},Rightarrow, k^4equiv 1, $ so $, color{#0a0}0equiv k(k^4)!+!3equiv color{#0a0}{k!+!3}$
therefore $ color{#0a0}{0equiv (3!+!k)}(3!-!k)equiv 9-color{#c00}{k^2}equiv 10, $ so $, bbox[5px,border:1px solid red]{k^2!+!1mid 10}$
Remark $ color{#c00}{k^2equiv -1},$ means $,k,$ behaves like $,i,$ so the above is essentially
$qquadqquadqquadquadbegin{align} &0equiv z equiv 3!+!i^5 equiv 3!+!i\[.4em]
Rightarrow &0 equiv zbar z equiv (3!+!i)(3!-!i) equiv 10end{align}$
which has a natural interpretation in Gaussian integer arithmetic (or complex numbers).
As for a "solution using polynomials", the above computation is generic and it translates to the following Euclidean gcd (or ideal) calculation with polynomials
$qquadqquad(x^2!+!1,!overbrace{x^5!+!3!!!}^{largecolor{#c00}{x^2 equiv -1}}), =, (!overbrace{x^2!+!1}^{largecolor{#0a0}{x equiv -3}},,x!+!3), = ,(10,,x!+!3)$
using Euclid's $, (a,,b) = (a,, bbmod a)., $ We could go further and extract the Bezout relation giving $10$ as a linear combination of $,x^2!+!1,,x^5!+!3,,$ but there is no need to do so for the problem at hand.
$endgroup$
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
1
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
1
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
|
show 1 more comment
$begingroup$
$!bmod k^2!+!1!:, color{#c00}{k^2equiv-1},Rightarrow, k^4equiv 1, $ so $, color{#0a0}0equiv k(k^4)!+!3equiv color{#0a0}{k!+!3}$
therefore $ color{#0a0}{0equiv (3!+!k)}(3!-!k)equiv 9-color{#c00}{k^2}equiv 10, $ so $, bbox[5px,border:1px solid red]{k^2!+!1mid 10}$
Remark $ color{#c00}{k^2equiv -1},$ means $,k,$ behaves like $,i,$ so the above is essentially
$qquadqquadqquadquadbegin{align} &0equiv z equiv 3!+!i^5 equiv 3!+!i\[.4em]
Rightarrow &0 equiv zbar z equiv (3!+!i)(3!-!i) equiv 10end{align}$
which has a natural interpretation in Gaussian integer arithmetic (or complex numbers).
As for a "solution using polynomials", the above computation is generic and it translates to the following Euclidean gcd (or ideal) calculation with polynomials
$qquadqquad(x^2!+!1,!overbrace{x^5!+!3!!!}^{largecolor{#c00}{x^2 equiv -1}}), =, (!overbrace{x^2!+!1}^{largecolor{#0a0}{x equiv -3}},,x!+!3), = ,(10,,x!+!3)$
using Euclid's $, (a,,b) = (a,, bbmod a)., $ We could go further and extract the Bezout relation giving $10$ as a linear combination of $,x^2!+!1,,x^5!+!3,,$ but there is no need to do so for the problem at hand.
$endgroup$
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
1
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
1
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
|
show 1 more comment
$begingroup$
$!bmod k^2!+!1!:, color{#c00}{k^2equiv-1},Rightarrow, k^4equiv 1, $ so $, color{#0a0}0equiv k(k^4)!+!3equiv color{#0a0}{k!+!3}$
therefore $ color{#0a0}{0equiv (3!+!k)}(3!-!k)equiv 9-color{#c00}{k^2}equiv 10, $ so $, bbox[5px,border:1px solid red]{k^2!+!1mid 10}$
Remark $ color{#c00}{k^2equiv -1},$ means $,k,$ behaves like $,i,$ so the above is essentially
$qquadqquadqquadquadbegin{align} &0equiv z equiv 3!+!i^5 equiv 3!+!i\[.4em]
Rightarrow &0 equiv zbar z equiv (3!+!i)(3!-!i) equiv 10end{align}$
which has a natural interpretation in Gaussian integer arithmetic (or complex numbers).
As for a "solution using polynomials", the above computation is generic and it translates to the following Euclidean gcd (or ideal) calculation with polynomials
$qquadqquad(x^2!+!1,!overbrace{x^5!+!3!!!}^{largecolor{#c00}{x^2 equiv -1}}), =, (!overbrace{x^2!+!1}^{largecolor{#0a0}{x equiv -3}},,x!+!3), = ,(10,,x!+!3)$
using Euclid's $, (a,,b) = (a,, bbmod a)., $ We could go further and extract the Bezout relation giving $10$ as a linear combination of $,x^2!+!1,,x^5!+!3,,$ but there is no need to do so for the problem at hand.
$endgroup$
$!bmod k^2!+!1!:, color{#c00}{k^2equiv-1},Rightarrow, k^4equiv 1, $ so $, color{#0a0}0equiv k(k^4)!+!3equiv color{#0a0}{k!+!3}$
therefore $ color{#0a0}{0equiv (3!+!k)}(3!-!k)equiv 9-color{#c00}{k^2}equiv 10, $ so $, bbox[5px,border:1px solid red]{k^2!+!1mid 10}$
Remark $ color{#c00}{k^2equiv -1},$ means $,k,$ behaves like $,i,$ so the above is essentially
$qquadqquadqquadquadbegin{align} &0equiv z equiv 3!+!i^5 equiv 3!+!i\[.4em]
Rightarrow &0 equiv zbar z equiv (3!+!i)(3!-!i) equiv 10end{align}$
which has a natural interpretation in Gaussian integer arithmetic (or complex numbers).
As for a "solution using polynomials", the above computation is generic and it translates to the following Euclidean gcd (or ideal) calculation with polynomials
$qquadqquad(x^2!+!1,!overbrace{x^5!+!3!!!}^{largecolor{#c00}{x^2 equiv -1}}), =, (!overbrace{x^2!+!1}^{largecolor{#0a0}{x equiv -3}},,x!+!3), = ,(10,,x!+!3)$
using Euclid's $, (a,,b) = (a,, bbmod a)., $ We could go further and extract the Bezout relation giving $10$ as a linear combination of $,x^2!+!1,,x^5!+!3,,$ but there is no need to do so for the problem at hand.
edited Nov 30 '18 at 1:19
answered Nov 29 '18 at 15:56
Bill DubuqueBill Dubuque
209k29191633
209k29191633
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
1
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
1
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
|
show 1 more comment
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
1
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
1
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
You wrote a solution which alrady exsist
$endgroup$
– greedoid
Nov 29 '18 at 15:59
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
@greedoid I don't agree (and the other answers weren't there when I loaded this page)
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:00
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
$begingroup$
Why is that........?
$endgroup$
– greedoid
Nov 29 '18 at 16:01
1
1
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
$begingroup$
$k^2equiv-1$ is really a neat idea, can this be generalized for divisibility by arbitrary 2-polynomials, not only this with almost trivial roots $pm i$?
$endgroup$
– kirilloid
Nov 29 '18 at 16:26
1
1
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
$begingroup$
@kirilloid Yes, $bmod x^2!-a,$ we can use $,x^2equiv a$ to reduce every polynomial $f(x)$ to a congruent linear polynomial $r(x)$. It's essentially an efficient what to compute the remainder $r(x)$ on dividing $f(x)$ by $,x^2!-a.,$ Since $,f(x) = q(x) (x^2!-a) + r(x),$ we have $,x^2!-amid f(x)iff x^2!-amid r(x),,$ so to test divisibility we need only the remainder (the quotient plays no role so it is a wasteful to compute it). The same works not only for $,x^2-a,$ but for any polynomial. But in the general case the modular arithmetic is more complicated.
$endgroup$
– Bill Dubuque
Nov 29 '18 at 16:49
|
show 1 more comment
$begingroup$
Solution with substitutions.
If $k^5+3$ is divisible by $k^2+1$, it could be expressed as $(k^2+1)(k^3+n)$, which if expanded, leads us to $k^3+nk^2+n-3$.
Now let's make another substitution by introducing another variable $a: k=-(n+a)$, which will get us $-an^2+(1-2a^2)n-(a^3+3)$. It is solvable for $n$ only when $Dge0$ or $-4a^2-12a+1ge0$.
Roots of the last [expression turned into an] equation are $-3pmfrac{sqrt{10}}{2}$, and since we are looking only for integers, we need to check $ain{-3,-2,-1,0}$.
Now we can start unrolling our substitutions. For all valid values of $a$, solve $-an^2+(1-2a^2)n-(a^3+3)$ for $n$, and get values of $k$ directly as $-(n+a)$
$$begin {array} {r|r} equation & a & n & k\
hline
3n^2-17n+24=0 & -3 & 3 & 0 \
3n^2-17n+24=0 & -3 & frac83 & - \
2n^2-7n+5=0 & -2 & 1 & 1 \
2n^2-7n+5=0 & -2 & 2.5 & - \
n^2-n-2=0 & -1 & -1 & 2 \
n^2-n-2=0 & -1 & 2 & -1 \
n-3=0 & 0 & 3 & -3 \
end {array}$$
$endgroup$
add a comment |
$begingroup$
Solution with substitutions.
If $k^5+3$ is divisible by $k^2+1$, it could be expressed as $(k^2+1)(k^3+n)$, which if expanded, leads us to $k^3+nk^2+n-3$.
Now let's make another substitution by introducing another variable $a: k=-(n+a)$, which will get us $-an^2+(1-2a^2)n-(a^3+3)$. It is solvable for $n$ only when $Dge0$ or $-4a^2-12a+1ge0$.
Roots of the last [expression turned into an] equation are $-3pmfrac{sqrt{10}}{2}$, and since we are looking only for integers, we need to check $ain{-3,-2,-1,0}$.
Now we can start unrolling our substitutions. For all valid values of $a$, solve $-an^2+(1-2a^2)n-(a^3+3)$ for $n$, and get values of $k$ directly as $-(n+a)$
$$begin {array} {r|r} equation & a & n & k\
hline
3n^2-17n+24=0 & -3 & 3 & 0 \
3n^2-17n+24=0 & -3 & frac83 & - \
2n^2-7n+5=0 & -2 & 1 & 1 \
2n^2-7n+5=0 & -2 & 2.5 & - \
n^2-n-2=0 & -1 & -1 & 2 \
n^2-n-2=0 & -1 & 2 & -1 \
n-3=0 & 0 & 3 & -3 \
end {array}$$
$endgroup$
add a comment |
$begingroup$
Solution with substitutions.
If $k^5+3$ is divisible by $k^2+1$, it could be expressed as $(k^2+1)(k^3+n)$, which if expanded, leads us to $k^3+nk^2+n-3$.
Now let's make another substitution by introducing another variable $a: k=-(n+a)$, which will get us $-an^2+(1-2a^2)n-(a^3+3)$. It is solvable for $n$ only when $Dge0$ or $-4a^2-12a+1ge0$.
Roots of the last [expression turned into an] equation are $-3pmfrac{sqrt{10}}{2}$, and since we are looking only for integers, we need to check $ain{-3,-2,-1,0}$.
Now we can start unrolling our substitutions. For all valid values of $a$, solve $-an^2+(1-2a^2)n-(a^3+3)$ for $n$, and get values of $k$ directly as $-(n+a)$
$$begin {array} {r|r} equation & a & n & k\
hline
3n^2-17n+24=0 & -3 & 3 & 0 \
3n^2-17n+24=0 & -3 & frac83 & - \
2n^2-7n+5=0 & -2 & 1 & 1 \
2n^2-7n+5=0 & -2 & 2.5 & - \
n^2-n-2=0 & -1 & -1 & 2 \
n^2-n-2=0 & -1 & 2 & -1 \
n-3=0 & 0 & 3 & -3 \
end {array}$$
$endgroup$
Solution with substitutions.
If $k^5+3$ is divisible by $k^2+1$, it could be expressed as $(k^2+1)(k^3+n)$, which if expanded, leads us to $k^3+nk^2+n-3$.
Now let's make another substitution by introducing another variable $a: k=-(n+a)$, which will get us $-an^2+(1-2a^2)n-(a^3+3)$. It is solvable for $n$ only when $Dge0$ or $-4a^2-12a+1ge0$.
Roots of the last [expression turned into an] equation are $-3pmfrac{sqrt{10}}{2}$, and since we are looking only for integers, we need to check $ain{-3,-2,-1,0}$.
Now we can start unrolling our substitutions. For all valid values of $a$, solve $-an^2+(1-2a^2)n-(a^3+3)$ for $n$, and get values of $k$ directly as $-(n+a)$
$$begin {array} {r|r} equation & a & n & k\
hline
3n^2-17n+24=0 & -3 & 3 & 0 \
3n^2-17n+24=0 & -3 & frac83 & - \
2n^2-7n+5=0 & -2 & 1 & 1 \
2n^2-7n+5=0 & -2 & 2.5 & - \
n^2-n-2=0 & -1 & -1 & 2 \
n^2-n-2=0 & -1 & 2 & -1 \
n-3=0 & 0 & 3 & -3 \
end {array}$$
edited Nov 29 '18 at 16:27
answered Nov 29 '18 at 15:41
kirilloidkirilloid
1656
1656
add a comment |
add a comment |
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