Value of Vandermonde type determinant
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Let $x_1,...,x_n $ are distinct real numbers.
Is it a formula for the Vandermonde type determinant $V(x_1, cdots,x_n)$ whose last column is $x_1^k, cdots, x_n^k$, where $k geq n$, instead of $x_1^{n-1}, cdots, x_n^{n-1}$?
Thanks
linear-algebra determinant
$endgroup$
add a comment |
$begingroup$
Let $x_1,...,x_n $ are distinct real numbers.
Is it a formula for the Vandermonde type determinant $V(x_1, cdots,x_n)$ whose last column is $x_1^k, cdots, x_n^k$, where $k geq n$, instead of $x_1^{n-1}, cdots, x_n^{n-1}$?
Thanks
linear-algebra determinant
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$begingroup$
All the other columns are the same as usual?
$endgroup$
– EuYu
Nov 16 '12 at 15:01
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Yes, the first $n-1$ column are the same.
$endgroup$
– Richard
Nov 16 '12 at 15:01
add a comment |
$begingroup$
Let $x_1,...,x_n $ are distinct real numbers.
Is it a formula for the Vandermonde type determinant $V(x_1, cdots,x_n)$ whose last column is $x_1^k, cdots, x_n^k$, where $k geq n$, instead of $x_1^{n-1}, cdots, x_n^{n-1}$?
Thanks
linear-algebra determinant
$endgroup$
Let $x_1,...,x_n $ are distinct real numbers.
Is it a formula for the Vandermonde type determinant $V(x_1, cdots,x_n)$ whose last column is $x_1^k, cdots, x_n^k$, where $k geq n$, instead of $x_1^{n-1}, cdots, x_n^{n-1}$?
Thanks
linear-algebra determinant
linear-algebra determinant
edited Nov 16 '12 at 15:18
uforoboa
4,89811829
4,89811829
asked Nov 16 '12 at 14:58
RichardRichard
1,77712046
1,77712046
$begingroup$
All the other columns are the same as usual?
$endgroup$
– EuYu
Nov 16 '12 at 15:01
$begingroup$
Yes, the first $n-1$ column are the same.
$endgroup$
– Richard
Nov 16 '12 at 15:01
add a comment |
$begingroup$
All the other columns are the same as usual?
$endgroup$
– EuYu
Nov 16 '12 at 15:01
$begingroup$
Yes, the first $n-1$ column are the same.
$endgroup$
– Richard
Nov 16 '12 at 15:01
$begingroup$
All the other columns are the same as usual?
$endgroup$
– EuYu
Nov 16 '12 at 15:01
$begingroup$
All the other columns are the same as usual?
$endgroup$
– EuYu
Nov 16 '12 at 15:01
$begingroup$
Yes, the first $n-1$ column are the same.
$endgroup$
– Richard
Nov 16 '12 at 15:01
$begingroup$
Yes, the first $n-1$ column are the same.
$endgroup$
– Richard
Nov 16 '12 at 15:01
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
Sure, at least you can find such a formula for any fixed $k geqslant n$. Not sure about a general formula for an unknown $k$ though.
Here is a hint: if all the $x_i$ are distinct, vector $(x_1^k,ldots,x_n^k)$ is a linear combination of vectors $(x_1^i,ldots,x_n^i)$, $i=0,,1,ldots,n-1$. If
$$
(x_1^k,ldots,x_n^k) = sum_{i=0}^{n-1} lambda_i (x_1^i,ldots,x_n^i),
$$
then your determinant is simply equal to $lambda_{n-1}$ times the usual Vandermonde determinant. So all you need to do is find $lambda_{n-1}$.
$endgroup$
add a comment |
$begingroup$
To continue Dan's answer, we want
$$ x_i^k = sum_{j=0}^{n-1} lambda_j x_i^j, qquad 1 le i le n $$
That is the polynomial $p(x) := sum_{j=0}^{n-1} lambda_j x^j$ interpolates $x^k$ at $x_0, ldots, x_{n-1}$. Lagrange interpolation gives
$$ p(x) = sum_{j=0}^{n-1} x_j^k cdot prod_{ell ne j} frac{x-x_ell}{x_j - x_ell} $$
$lambda_{n-1}$ is the coefficient of $x^{n-1}$, so it is
$$ lambda_{n-1} = sum_{j=0}^{n-1} x_j^k prod_{ellne j} frac 1{x_j - x_ell}. $$
$endgroup$
add a comment |
$begingroup$
Perform Laplace expansion along the last column. As the deletion of the $ell$-th row and the last column gives a $(n-1)times(n-1)$ Vandermonde matrix (in the original flavour), we get
$$
sum_{ell=1}^n (-1)^{ell+n} x_ell^kprod_{i<j,textrm{ and },i,jnot=ell}(x_j-x_i).
$$
$endgroup$
add a comment |
$begingroup$
Actually there is a general formula but not an easy way to describe it.
Let me first "define" the polynomials $f_m(x_1,x_2,ldots,x_k)$ through some examples.
- $f_0(x_1,x_2,x_3)=1.$
- $f_1(x_1,x_2,x_3)=x_1+x_2+x_3.$
- $f_2(x_1,x_2,x_3)=x_1^2+x_2^2+x_3^2+x_1x_2+x_1x_3+x_2x_3.$
- $f_3(x_1,x_2,x_3)=x_1^3+x_2^3+x_3^3+x_1^2x_2+x_1^2x_3+x_2^2x_1+x_2^2x_3+x_3^2x_1+x_3^2x_2+x_1x_2x_3.$
- $f_3(x_1,x_2,x_3,x_4)=x_1^3+x_1^2x_2+x_1^2x_3+x_1^2x_4+x_1x_2^2+x_1x_2x_3+x_1x_2x_4+x_1x_3^2+x_1x_3x_4+x_1x_4^2+x_2^3+x_2^2x_3+x_2^2x_4+x_2x_3^2+x_2x_3x_4+x_2x_4^2+x_3^3+x_3^2x_4+x_3x_4^2+x_4^3
.$ - $ldots$
Let $V(x_1,x_2,ldots,x_n,k)$ denote the Vandermonde type determinant whose last column is $x_1^k,x_2^k,ldots,x_n^k$ where $kgeq n-1$. So $V(x_1,x_2,ldots,x_n,n-1)$ is the usual Vandermonde. Then $$V(x_1,x_2,ldots,x_n,k)=V(x_1,x_2,ldots,x_n,n-1)cdot f_{k-n+1}(x_1,x_2,ldots,x_n).$$
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
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active
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active
oldest
votes
$begingroup$
Sure, at least you can find such a formula for any fixed $k geqslant n$. Not sure about a general formula for an unknown $k$ though.
Here is a hint: if all the $x_i$ are distinct, vector $(x_1^k,ldots,x_n^k)$ is a linear combination of vectors $(x_1^i,ldots,x_n^i)$, $i=0,,1,ldots,n-1$. If
$$
(x_1^k,ldots,x_n^k) = sum_{i=0}^{n-1} lambda_i (x_1^i,ldots,x_n^i),
$$
then your determinant is simply equal to $lambda_{n-1}$ times the usual Vandermonde determinant. So all you need to do is find $lambda_{n-1}$.
$endgroup$
add a comment |
$begingroup$
Sure, at least you can find such a formula for any fixed $k geqslant n$. Not sure about a general formula for an unknown $k$ though.
Here is a hint: if all the $x_i$ are distinct, vector $(x_1^k,ldots,x_n^k)$ is a linear combination of vectors $(x_1^i,ldots,x_n^i)$, $i=0,,1,ldots,n-1$. If
$$
(x_1^k,ldots,x_n^k) = sum_{i=0}^{n-1} lambda_i (x_1^i,ldots,x_n^i),
$$
then your determinant is simply equal to $lambda_{n-1}$ times the usual Vandermonde determinant. So all you need to do is find $lambda_{n-1}$.
$endgroup$
add a comment |
$begingroup$
Sure, at least you can find such a formula for any fixed $k geqslant n$. Not sure about a general formula for an unknown $k$ though.
Here is a hint: if all the $x_i$ are distinct, vector $(x_1^k,ldots,x_n^k)$ is a linear combination of vectors $(x_1^i,ldots,x_n^i)$, $i=0,,1,ldots,n-1$. If
$$
(x_1^k,ldots,x_n^k) = sum_{i=0}^{n-1} lambda_i (x_1^i,ldots,x_n^i),
$$
then your determinant is simply equal to $lambda_{n-1}$ times the usual Vandermonde determinant. So all you need to do is find $lambda_{n-1}$.
$endgroup$
Sure, at least you can find such a formula for any fixed $k geqslant n$. Not sure about a general formula for an unknown $k$ though.
Here is a hint: if all the $x_i$ are distinct, vector $(x_1^k,ldots,x_n^k)$ is a linear combination of vectors $(x_1^i,ldots,x_n^i)$, $i=0,,1,ldots,n-1$. If
$$
(x_1^k,ldots,x_n^k) = sum_{i=0}^{n-1} lambda_i (x_1^i,ldots,x_n^i),
$$
then your determinant is simply equal to $lambda_{n-1}$ times the usual Vandermonde determinant. So all you need to do is find $lambda_{n-1}$.
answered Nov 16 '12 at 15:17
Dan ShvedDan Shved
12.7k2045
12.7k2045
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add a comment |
$begingroup$
To continue Dan's answer, we want
$$ x_i^k = sum_{j=0}^{n-1} lambda_j x_i^j, qquad 1 le i le n $$
That is the polynomial $p(x) := sum_{j=0}^{n-1} lambda_j x^j$ interpolates $x^k$ at $x_0, ldots, x_{n-1}$. Lagrange interpolation gives
$$ p(x) = sum_{j=0}^{n-1} x_j^k cdot prod_{ell ne j} frac{x-x_ell}{x_j - x_ell} $$
$lambda_{n-1}$ is the coefficient of $x^{n-1}$, so it is
$$ lambda_{n-1} = sum_{j=0}^{n-1} x_j^k prod_{ellne j} frac 1{x_j - x_ell}. $$
$endgroup$
add a comment |
$begingroup$
To continue Dan's answer, we want
$$ x_i^k = sum_{j=0}^{n-1} lambda_j x_i^j, qquad 1 le i le n $$
That is the polynomial $p(x) := sum_{j=0}^{n-1} lambda_j x^j$ interpolates $x^k$ at $x_0, ldots, x_{n-1}$. Lagrange interpolation gives
$$ p(x) = sum_{j=0}^{n-1} x_j^k cdot prod_{ell ne j} frac{x-x_ell}{x_j - x_ell} $$
$lambda_{n-1}$ is the coefficient of $x^{n-1}$, so it is
$$ lambda_{n-1} = sum_{j=0}^{n-1} x_j^k prod_{ellne j} frac 1{x_j - x_ell}. $$
$endgroup$
add a comment |
$begingroup$
To continue Dan's answer, we want
$$ x_i^k = sum_{j=0}^{n-1} lambda_j x_i^j, qquad 1 le i le n $$
That is the polynomial $p(x) := sum_{j=0}^{n-1} lambda_j x^j$ interpolates $x^k$ at $x_0, ldots, x_{n-1}$. Lagrange interpolation gives
$$ p(x) = sum_{j=0}^{n-1} x_j^k cdot prod_{ell ne j} frac{x-x_ell}{x_j - x_ell} $$
$lambda_{n-1}$ is the coefficient of $x^{n-1}$, so it is
$$ lambda_{n-1} = sum_{j=0}^{n-1} x_j^k prod_{ellne j} frac 1{x_j - x_ell}. $$
$endgroup$
To continue Dan's answer, we want
$$ x_i^k = sum_{j=0}^{n-1} lambda_j x_i^j, qquad 1 le i le n $$
That is the polynomial $p(x) := sum_{j=0}^{n-1} lambda_j x^j$ interpolates $x^k$ at $x_0, ldots, x_{n-1}$. Lagrange interpolation gives
$$ p(x) = sum_{j=0}^{n-1} x_j^k cdot prod_{ell ne j} frac{x-x_ell}{x_j - x_ell} $$
$lambda_{n-1}$ is the coefficient of $x^{n-1}$, so it is
$$ lambda_{n-1} = sum_{j=0}^{n-1} x_j^k prod_{ellne j} frac 1{x_j - x_ell}. $$
answered Nov 16 '12 at 15:29
martinimartini
70.6k45991
70.6k45991
add a comment |
add a comment |
$begingroup$
Perform Laplace expansion along the last column. As the deletion of the $ell$-th row and the last column gives a $(n-1)times(n-1)$ Vandermonde matrix (in the original flavour), we get
$$
sum_{ell=1}^n (-1)^{ell+n} x_ell^kprod_{i<j,textrm{ and },i,jnot=ell}(x_j-x_i).
$$
$endgroup$
add a comment |
$begingroup$
Perform Laplace expansion along the last column. As the deletion of the $ell$-th row and the last column gives a $(n-1)times(n-1)$ Vandermonde matrix (in the original flavour), we get
$$
sum_{ell=1}^n (-1)^{ell+n} x_ell^kprod_{i<j,textrm{ and },i,jnot=ell}(x_j-x_i).
$$
$endgroup$
add a comment |
$begingroup$
Perform Laplace expansion along the last column. As the deletion of the $ell$-th row and the last column gives a $(n-1)times(n-1)$ Vandermonde matrix (in the original flavour), we get
$$
sum_{ell=1}^n (-1)^{ell+n} x_ell^kprod_{i<j,textrm{ and },i,jnot=ell}(x_j-x_i).
$$
$endgroup$
Perform Laplace expansion along the last column. As the deletion of the $ell$-th row and the last column gives a $(n-1)times(n-1)$ Vandermonde matrix (in the original flavour), we get
$$
sum_{ell=1}^n (-1)^{ell+n} x_ell^kprod_{i<j,textrm{ and },i,jnot=ell}(x_j-x_i).
$$
answered Nov 16 '12 at 15:39
user1551user1551
73.5k566129
73.5k566129
add a comment |
add a comment |
$begingroup$
Actually there is a general formula but not an easy way to describe it.
Let me first "define" the polynomials $f_m(x_1,x_2,ldots,x_k)$ through some examples.
- $f_0(x_1,x_2,x_3)=1.$
- $f_1(x_1,x_2,x_3)=x_1+x_2+x_3.$
- $f_2(x_1,x_2,x_3)=x_1^2+x_2^2+x_3^2+x_1x_2+x_1x_3+x_2x_3.$
- $f_3(x_1,x_2,x_3)=x_1^3+x_2^3+x_3^3+x_1^2x_2+x_1^2x_3+x_2^2x_1+x_2^2x_3+x_3^2x_1+x_3^2x_2+x_1x_2x_3.$
- $f_3(x_1,x_2,x_3,x_4)=x_1^3+x_1^2x_2+x_1^2x_3+x_1^2x_4+x_1x_2^2+x_1x_2x_3+x_1x_2x_4+x_1x_3^2+x_1x_3x_4+x_1x_4^2+x_2^3+x_2^2x_3+x_2^2x_4+x_2x_3^2+x_2x_3x_4+x_2x_4^2+x_3^3+x_3^2x_4+x_3x_4^2+x_4^3
.$ - $ldots$
Let $V(x_1,x_2,ldots,x_n,k)$ denote the Vandermonde type determinant whose last column is $x_1^k,x_2^k,ldots,x_n^k$ where $kgeq n-1$. So $V(x_1,x_2,ldots,x_n,n-1)$ is the usual Vandermonde. Then $$V(x_1,x_2,ldots,x_n,k)=V(x_1,x_2,ldots,x_n,n-1)cdot f_{k-n+1}(x_1,x_2,ldots,x_n).$$
$endgroup$
add a comment |
$begingroup$
Actually there is a general formula but not an easy way to describe it.
Let me first "define" the polynomials $f_m(x_1,x_2,ldots,x_k)$ through some examples.
- $f_0(x_1,x_2,x_3)=1.$
- $f_1(x_1,x_2,x_3)=x_1+x_2+x_3.$
- $f_2(x_1,x_2,x_3)=x_1^2+x_2^2+x_3^2+x_1x_2+x_1x_3+x_2x_3.$
- $f_3(x_1,x_2,x_3)=x_1^3+x_2^3+x_3^3+x_1^2x_2+x_1^2x_3+x_2^2x_1+x_2^2x_3+x_3^2x_1+x_3^2x_2+x_1x_2x_3.$
- $f_3(x_1,x_2,x_3,x_4)=x_1^3+x_1^2x_2+x_1^2x_3+x_1^2x_4+x_1x_2^2+x_1x_2x_3+x_1x_2x_4+x_1x_3^2+x_1x_3x_4+x_1x_4^2+x_2^3+x_2^2x_3+x_2^2x_4+x_2x_3^2+x_2x_3x_4+x_2x_4^2+x_3^3+x_3^2x_4+x_3x_4^2+x_4^3
.$ - $ldots$
Let $V(x_1,x_2,ldots,x_n,k)$ denote the Vandermonde type determinant whose last column is $x_1^k,x_2^k,ldots,x_n^k$ where $kgeq n-1$. So $V(x_1,x_2,ldots,x_n,n-1)$ is the usual Vandermonde. Then $$V(x_1,x_2,ldots,x_n,k)=V(x_1,x_2,ldots,x_n,n-1)cdot f_{k-n+1}(x_1,x_2,ldots,x_n).$$
$endgroup$
add a comment |
$begingroup$
Actually there is a general formula but not an easy way to describe it.
Let me first "define" the polynomials $f_m(x_1,x_2,ldots,x_k)$ through some examples.
- $f_0(x_1,x_2,x_3)=1.$
- $f_1(x_1,x_2,x_3)=x_1+x_2+x_3.$
- $f_2(x_1,x_2,x_3)=x_1^2+x_2^2+x_3^2+x_1x_2+x_1x_3+x_2x_3.$
- $f_3(x_1,x_2,x_3)=x_1^3+x_2^3+x_3^3+x_1^2x_2+x_1^2x_3+x_2^2x_1+x_2^2x_3+x_3^2x_1+x_3^2x_2+x_1x_2x_3.$
- $f_3(x_1,x_2,x_3,x_4)=x_1^3+x_1^2x_2+x_1^2x_3+x_1^2x_4+x_1x_2^2+x_1x_2x_3+x_1x_2x_4+x_1x_3^2+x_1x_3x_4+x_1x_4^2+x_2^3+x_2^2x_3+x_2^2x_4+x_2x_3^2+x_2x_3x_4+x_2x_4^2+x_3^3+x_3^2x_4+x_3x_4^2+x_4^3
.$ - $ldots$
Let $V(x_1,x_2,ldots,x_n,k)$ denote the Vandermonde type determinant whose last column is $x_1^k,x_2^k,ldots,x_n^k$ where $kgeq n-1$. So $V(x_1,x_2,ldots,x_n,n-1)$ is the usual Vandermonde. Then $$V(x_1,x_2,ldots,x_n,k)=V(x_1,x_2,ldots,x_n,n-1)cdot f_{k-n+1}(x_1,x_2,ldots,x_n).$$
$endgroup$
Actually there is a general formula but not an easy way to describe it.
Let me first "define" the polynomials $f_m(x_1,x_2,ldots,x_k)$ through some examples.
- $f_0(x_1,x_2,x_3)=1.$
- $f_1(x_1,x_2,x_3)=x_1+x_2+x_3.$
- $f_2(x_1,x_2,x_3)=x_1^2+x_2^2+x_3^2+x_1x_2+x_1x_3+x_2x_3.$
- $f_3(x_1,x_2,x_3)=x_1^3+x_2^3+x_3^3+x_1^2x_2+x_1^2x_3+x_2^2x_1+x_2^2x_3+x_3^2x_1+x_3^2x_2+x_1x_2x_3.$
- $f_3(x_1,x_2,x_3,x_4)=x_1^3+x_1^2x_2+x_1^2x_3+x_1^2x_4+x_1x_2^2+x_1x_2x_3+x_1x_2x_4+x_1x_3^2+x_1x_3x_4+x_1x_4^2+x_2^3+x_2^2x_3+x_2^2x_4+x_2x_3^2+x_2x_3x_4+x_2x_4^2+x_3^3+x_3^2x_4+x_3x_4^2+x_4^3
.$ - $ldots$
Let $V(x_1,x_2,ldots,x_n,k)$ denote the Vandermonde type determinant whose last column is $x_1^k,x_2^k,ldots,x_n^k$ where $kgeq n-1$. So $V(x_1,x_2,ldots,x_n,n-1)$ is the usual Vandermonde. Then $$V(x_1,x_2,ldots,x_n,k)=V(x_1,x_2,ldots,x_n,n-1)cdot f_{k-n+1}(x_1,x_2,ldots,x_n).$$
edited Jan 21 at 18:52
answered Nov 16 '12 at 16:41
P..P..
13.5k22348
13.5k22348
add a comment |
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$begingroup$
All the other columns are the same as usual?
$endgroup$
– EuYu
Nov 16 '12 at 15:01
$begingroup$
Yes, the first $n-1$ column are the same.
$endgroup$
– Richard
Nov 16 '12 at 15:01