Skip to main content
formatting, added tag
Source Link
YCor
  • 63.9k
  • 5
  • 187
  • 286

Characterizing zeros of schurSchur functions over $\mathbb{R^n}$ or $\mathbb{C^n}$

Let $m_i\in \mathbb{N}, 1\leq i \leq n $ such that wlog if $m_i < m_j\in \mathbb{Z^+}$ then $i < j$. Consider the Vandermonde matrix $$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1} & \ldots & x_{n+1}^{m_1} \\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} & \ldots & x_{n+1}^{m_2} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_n} & x_2^{m_n} & x_3^{m_n} & \ldots & x_{n+1}^{m_n} \\ \end{bmatrix}$$ Under Whatwhat conditions is the generalized Vandermonde Matrix Invertiblematrix invertible? It is clear that if one lets $x_i=x_j$,then the determinant is Zerozero, and therefore the principle Vandermonde Determinant $V_p=\prod_{1 \leq i < j\leq n}^n(x_i-x_j)$ can be factored from the $det(V)$$\det(V)$. What remains is the schur function, which is a homogeneous polynomial of degree d which depends on $n$, which symmetric over its variables $x_i,1\leq i\leq n$. There is a proof of the fact that the coefficients of the schur function are positive integers which was given by Mitchel long ago.

For $n=3$ things become difficult already as $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1}\\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} \end{bmatrix}$$ $det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$$\det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$.

What are the conditions under which the generalized Vandermonde matrix is invertible over $\mathbb{R^n}$ or $\mathbb{C^n}$? Is there Hopehope?

Well yeahyes, a little bit.There There are some results one could see from Mitchel's result about the positivity of the coefficients of the Schur function. If we are working over $\mathbb{R^n}$, then as a consequence of the positivity of the coefficients mentioned by Mitchel, one can deduce that if $x_i$ are distinct and positive for $1 \leq i \leq n$, then the schurSchur function takes on positive values and therefore non-zero. This result is mentioned in this Paperpaper.

1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1 & x_2 & x_3\\ x_1^{3} & x_2^{3} & x_3^{3} \end{bmatrix}$$ where $m_1 = 1, m_2 = 3$, then $det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$$\det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$, so our schurSchur function is the elementary symmetric function $x_1+x_2+x_3$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

$det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$$\det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$

$det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$$\det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$, so our schurSchur function is $ (x_1+x_2)(x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

$det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$$\det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$ *

So our schurSchur function is everything that we have after dividing out by $V_p$. This function is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

My goal is to really answer the following interesting question, Let ${m_i}$, $i \in \mathbb{N}$ be an increasing sequence of distinct positive integers, then consider the schurSchur function obtained by dividing out the following Vandermonde Determinantdeterminant by the principle Vandermonde:

where $m_{i_j}$ are terms in the sequence ${m_i}$, $i \in \mathbb{N}$ such that $m_{i_{j_1}}< m_{i_{j_2}}$ implies $j_1 < j_2$. Let $\Gamma$ ={all possible $m_{i_j}, 1 \leq j \leq n $}, then each element of $\Gamma$ corresponds to a unique schurSchur function. Let $S_{\Gamma}$ ={schurSchur functions induced by $\Gamma$}. Now Let $s \in S_{\Gamma}$ and $Z_s$ ={zeros of $s$}, then I would like to show that $\cap_{s \in S_{\Gamma} }Z_s$ is empty, provided that there are no non-trivial relations between the variables of the schurSchur function, i.e. the $n$ variables of the schurSchur function form a free abelian group of rank $n$, i.e. if the product of the coordinates of a zero of the schurSchur function does not satisfy that condition then you should exclude it.

As pointed out by Emmanuel Briand in the comments, if the sequence $m_i$ contains $n$ consecutive integers, then the zeros of the corresponding schurSchur function are trivial as they are of the form $(0,\cdots , 1, \cdots , 0)$ where 1 is located in the $i^{th}$ position. As a special case, you can see that if $m_i$ contains $1,2,\cdots , n-1$, then the answer to the question above is positive, i.e. the $\cap_{s \in S_{\Gamma} }Z_s$ is empty.

Characterizing zeros of schur functions over $\mathbb{R^n}$ or $\mathbb{C^n}$

Let $m_i\in \mathbb{N}, 1\leq i \leq n $ such that wlog if $m_i < m_j\in \mathbb{Z^+}$ then $i < j$. Consider the Vandermonde matrix $$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1} & \ldots & x_{n+1}^{m_1} \\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} & \ldots & x_{n+1}^{m_2} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_n} & x_2^{m_n} & x_3^{m_n} & \ldots & x_{n+1}^{m_n} \\ \end{bmatrix}$$ Under What conditions is the generalized Vandermonde Matrix Invertible? It is clear that if one lets $x_i=x_j$,then the determinant is Zero, and therefore the principle Vandermonde Determinant $V_p=\prod_{1 \leq i < j\leq n}^n(x_i-x_j)$ can be factored from the $det(V)$. What remains is the schur function, which is a homogeneous polynomial of degree d which depends on $n$, which symmetric over its variables $x_i,1\leq i\leq n$. There is a proof of the fact that the coefficients of the schur function are positive integers which was given by Mitchel long ago.

For $n=3$ things become difficult already as $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1}\\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} \end{bmatrix}$$ $det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$.

What are the conditions under which the generalized Vandermonde matrix is invertible over $\mathbb{R^n}$ or $\mathbb{C^n}$? Is there Hope?

Well yeah, a little bit.There are some results one could see from Mitchel's result about the positivity of the coefficients of the Schur function. If we are working over $\mathbb{R^n}$, then as a consequence of the positivity of the coefficients mentioned by Mitchel, one can deduce that if $x_i$ are distinct and positive for $1 \leq i \leq n$, then the schur function takes on positive values and therefore non-zero. This result is mentioned in this Paper.

1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1 & x_2 & x_3\\ x_1^{3} & x_2^{3} & x_3^{3} \end{bmatrix}$$ where $m_1 = 1, m_2 = 3$, then $det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$, so our schur function is the elementary symmetric function $x_1+x_2+x_3$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

$det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$

$det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$, so our schur function is $ (x_1+x_2)(x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

$det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$ *

So our schur function is everything that we have after dividing out by $V_p$. This function is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

My goal is to really answer the following interesting question, Let ${m_i}$, $i \in \mathbb{N}$ be an increasing sequence of distinct positive integers, then consider the schur function obtained by dividing out the following Vandermonde Determinant by the principle Vandermonde:

where $m_{i_j}$ are terms in the sequence ${m_i}$, $i \in \mathbb{N}$ such that $m_{i_{j_1}}< m_{i_{j_2}}$ implies $j_1 < j_2$. Let $\Gamma$ ={all possible $m_{i_j}, 1 \leq j \leq n $}, then each element of $\Gamma$ corresponds to a unique schur function. Let $S_{\Gamma}$ ={schur functions induced by $\Gamma$}. Now Let $s \in S_{\Gamma}$ and $Z_s$ ={zeros of $s$}, then I would like to show that $\cap_{s \in S_{\Gamma} }Z_s$ is empty, provided that there are no non-trivial relations between the variables of the schur function, i.e. the $n$ variables of the schur function form a free abelian group of rank $n$, i.e. if the product of the coordinates of a zero of the schur function does not satisfy that condition then you should exclude it.

As pointed out by Emmanuel Briand in the comments, if the sequence $m_i$ contains $n$ consecutive integers, then the zeros of the corresponding schur function are trivial as they are of the form $(0,\cdots , 1, \cdots , 0)$ where 1 is located in the $i^{th}$ position. As a special case, you can see that if $m_i$ contains $1,2,\cdots , n-1$, then the answer to the question above is positive, i.e. the $\cap_{s \in S_{\Gamma} }Z_s$ is empty.

Characterizing zeros of Schur functions over $\mathbb{R^n}$ or $\mathbb{C^n}$

Let $m_i\in \mathbb{N}, 1\leq i \leq n $ such that wlog if $m_i < m_j\in \mathbb{Z^+}$ then $i < j$. Consider the Vandermonde matrix $$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1} & \ldots & x_{n+1}^{m_1} \\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} & \ldots & x_{n+1}^{m_2} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_n} & x_2^{m_n} & x_3^{m_n} & \ldots & x_{n+1}^{m_n} \\ \end{bmatrix}$$ Under what conditions is the generalized Vandermonde matrix invertible? It is clear that if one lets $x_i=x_j$,then the determinant is zero, and therefore the principle Vandermonde Determinant $V_p=\prod_{1 \leq i < j\leq n}^n(x_i-x_j)$ can be factored from the $\det(V)$. What remains is the schur function, which is a homogeneous polynomial of degree d which depends on $n$, which symmetric over its variables $x_i,1\leq i\leq n$. There is a proof of the fact that the coefficients of the schur function are positive integers which was given by Mitchel long ago.

For $n=3$ things become difficult already as $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1}\\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} \end{bmatrix}$$ $\det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$.

What are the conditions under which the generalized Vandermonde matrix is invertible over $\mathbb{R^n}$ or $\mathbb{C^n}$? Is there hope?

Well yes, a little bit. There are some results one could see from Mitchel's result about the positivity of the coefficients of the Schur function. If we are working over $\mathbb{R^n}$, then as a consequence of the positivity of the coefficients mentioned by Mitchel, one can deduce that if $x_i$ are distinct and positive for $1 \leq i \leq n$, then the Schur function takes on positive values and therefore non-zero. This result is mentioned in this paper.

1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1 & x_2 & x_3\\ x_1^{3} & x_2^{3} & x_3^{3} \end{bmatrix}$$ where $m_1 = 1, m_2 = 3$, then $\det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$, so our Schur function is the elementary symmetric function $x_1+x_2+x_3$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

$\det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$

$\det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$, so our Schur function is $ (x_1+x_2)(x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

$\det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$ *

So our Schur function is everything that we have after dividing out by $V_p$. This function is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

My goal is to really answer the following interesting question, Let ${m_i}$, $i \in \mathbb{N}$ be an increasing sequence of distinct positive integers, then consider the Schur function obtained by dividing out the following Vandermonde determinant by the principle Vandermonde:

where $m_{i_j}$ are terms in the sequence ${m_i}$, $i \in \mathbb{N}$ such that $m_{i_{j_1}}< m_{i_{j_2}}$ implies $j_1 < j_2$. Let $\Gamma$ ={all possible $m_{i_j}, 1 \leq j \leq n $}, then each element of $\Gamma$ corresponds to a unique Schur function. Let $S_{\Gamma}$ ={Schur functions induced by $\Gamma$}. Now Let $s \in S_{\Gamma}$ and $Z_s$ ={zeros of $s$}, then I would like to show that $\cap_{s \in S_{\Gamma} }Z_s$ is empty, provided that there are no non-trivial relations between the variables of the Schur function, i.e. the $n$ variables of the Schur function form a free abelian group of rank $n$, i.e. if the product of the coordinates of a zero of the Schur function does not satisfy that condition then you should exclude it.

As pointed out by Emmanuel Briand in the comments, if the sequence $m_i$ contains $n$ consecutive integers, then the zeros of the corresponding Schur function are trivial as they are of the form $(0,\cdots , 1, \cdots , 0)$ where 1 is located in the $i^{th}$ position. As a special case, you can see that if $m_i$ contains $1,2,\cdots , n-1$, then the answer to the question above is positive, i.e. $\cap_{s \in S_{\Gamma} }Z_s$ is empty.

added 450 characters in body
Source Link

As pointed out by Emmanuel Briand in the comments, if the sequence $m_i$ contains $n$ consecutive integers, then the zeros of the corresponding schur function are trivial as they are of the form $(0,\cdots , 1, \cdots , 0)$ where 1 is located in the $i^{th}$ position. As a special case, you can see that if $m_i$ contains $1,2,\cdots , n-1$, then the answer to the question above is positive, i.e. the $\cap_{s \in S_{\Gamma} }Z_s$ is empty.

As pointed out by Emmanuel Briand in the comments, if the sequence $m_i$ contains $n$ consecutive integers, then the zeros of the corresponding schur function are trivial as they are of the form $(0,\cdots , 1, \cdots , 0)$ where 1 is located in the $i^{th}$ position. As a special case, you can see that if $m_i$ contains $1,2,\cdots , n-1$, then the answer to the question above is positive, i.e. the $\cap_{s \in S_{\Gamma} }Z_s$ is empty.

added 254 characters in body; added 57 characters in body; deleted 2 characters in body
Source Link

Let $m_i\in \mathbb{N}, 1\leq i \leq n $ such that wlog if $m_i < m_j\in \mathbb{Z^+}$ then $i < j$. Consider the Vandermonde matrix $$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1} & \ldots & x_{n+1}^{m_1} \\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} & \ldots & x_{n+1}^{m_2} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_n} & x_2^{m_n} & x_3^{m_n} & \ldots & x_{n+1}^{m_n} \\ \end{bmatrix}$$ Under What conditions is the generalized Vandermonde Matrix Invertible? It is clear that if one lets $x_i=x_j$,then the determinant is Zero, and therefore the principle Vandermonde Determinant $V_p=\prod_{1 \leq i < j\leq n}^n(x_i-x_j)$ can be factored from the $det(V)$. What remains is the schur function, which is a homogeneous polynomial of degree d which depends on $n$, which symmetric over its variables $x_i,1\leq i\leq n$. There is a proof of the fact that the coefficients of the schur function are positive integers which was given by Mitchel long ago.

When one looks at the simplest case where $x_i \in \mathbb{C}, 1\leq i \leq n$ and $n=2$, $$V= \begin{bmatrix} 1 & 1\\ x_1^n & x_2^n\\ \end{bmatrix}$$ one sees that the Vandermonde determinant is invertible when $\frac{x_1}{x_2}$ is not an $n^{th}$ root of unity.

For $n=3$ things become difficult already as $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1}\\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} \end{bmatrix}$$ $det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$.

As far as I see (which is not very far), characterizing the zero set of the this polynomial is not at all simple in either $\mathbb{C^3}$ or $\mathbb{R^3}$ with exception of the trivial case we already mentioned where $x_i=x_j$ for some $i\neq j$ and perhaps some roots of unity. One can guess that over $\mathbb{C^n}$, if the $|x_i|$ are distinct and have no-non trivial relations, then the schur function is non-zero. Which brings me back to my question:

What are the conditions under which the generalized Vandermonde matrix is invertible over $\mathbb{R^n}$ or $\mathbb{C^n}$? Is there Hope?

Well yeah, a little bit.There are some results one could see from Mitchel's result about the positivity of the coefficients of the Schur function. If we are working over $\mathbb{R^n}$, then as a consequence of the positivity of the coefficients mentioned by Mitchel, one can deduce that if $x_i$ are distinct and positive for $1 \leq i \leq n$, then the schur function takes on positive values and therefore non-zero. This result is mentioned in this Paper.

Examples:

1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1 & x_2 & x_3\\ x_1^{3} & x_2^{3} & x_3^{3} \end{bmatrix}$$ where $m_1 = 1, m_2 = 3$, then $det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$, so our schur function is the elementary symmetric function $x_1+x_2+x_3$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

  1. 1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{4} & x_2^{4} & x_3^{4}\\ x_1^{6} & x_2^{6} & x_3^{6} \end{bmatrix}$$ where $m_1 = 4, m_2 = 6$, then

$det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$

$det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$, so our schur function is $ (x_1+x_2)(x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

  1. 1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{4} & x_2^{4} & x_3^{4}\\ x_1^{7} & x_2^{7} & x_3^{7} \end{bmatrix}$$ where $m_1 = 4, m_2 = 7$, then

$det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$ *

$(x_2^3 x_1^5+x_3^3 x_1^5+x_2 x_3^2 x_1^5+x_2^2 x_3 x_1^5+x_2^4 x_1^4+x_3^4 x_1^4+$ $2 x_2 x_3^3 x_1^4+2 x_2^2 x_3^2 x_1^4+2 x_2^3 x_3 x_1^4+x_2^5 x_1^3+x_3^5 x_1^3+$ $2 x_2 x_3^4 x_1^3+3 x_2^2 x_3^3 x_1^3+3 x_2^3 x_3^2 x_1^3+2 x_2^4 x_3 x_1^3+x_2 x_3^5$ $x_1^2+2 x_2^2 x_3^4 x_1^2+3 x_2^3 x_3^3 x_1^2+2 x_2^4 x_3^2 x_1^2+x_2^5 x_3 x_1^2+$ $x_2^2 x_3^5 x_1+2 x_2^3 x_3^4 x_1+2 x_2^4 x_3^3 x_1+x_2^5 x_3^2 x_1+x_2^3 x_3^5+$ $x_2^4 x_3^4+x_2^5 x_3^3)$

So our schur function is everything that we have after dividing out by $V_p$. This function is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

My goal is to really answer the following interesting question, Let ${m_i}$, $i \in \mathbb{N}$ be an increasing sequence of distinct positive integers, then consider the schur function obtained by dividing out the following Vandermonde Determinant by the principle Vandermonde:

$$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_{i_1}} & x_2^{m_{i_1}} & x_3^{m_{i_1}} & \ldots & x_{n+1}^{m_{i_1}} \\ x_1^{m_{i_2}} & x_2^{m_{i_2}} & x_3^{m_{i_2}} & \ldots & x_{n+1}^{m_{i_2}} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_{i_n}} & x_2^{m_{i_n}} & x_3^{m_{i_n}} & \ldots & x_{n+1}^{m_{i_n}} \\ \end{bmatrix}$$

where $m_{i_j}$ are terms in the sequence ${m_i}$, $i \in \mathbb{N}$ such that $m_{i_{j_1}}< m_{i_{j_2}}$ implies $j_1 < j_2$. Let $\Gamma$ ={all possible $m_{i_j}, 1 \leq j \leq n $}, then each element of $\Gamma$ corresponds to a unique schur function. Let $S_{\Gamma}$ ={schur functions induced by $\Gamma$}. Now Let $s \in S_{\Gamma}$ and $Z_s$ ={zeros of $s$}, then I would like to show that $\cap_{s \in S_{\Gamma} }Z_s$ is empty, provided that there are no non-trivial relations between the variables of the schur function, i.e. the $n$ variables of the schur function form a free abelian group of rank $n$, i.e. if the product of the coordinates of a zero of the schur function does not satisfy that condition then you should exclude it.

Let $m_i\in \mathbb{N}, 1\leq i \leq n $ such that wlog if $m_i < m_j\in \mathbb{Z^+}$ then $i < j$. Consider the Vandermonde matrix $$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1} & \ldots & x_{n+1}^{m_1} \\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} & \ldots & x_{n+1}^{m_2} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_n} & x_2^{m_n} & x_3^{m_n} & \ldots & x_{n+1}^{m_n} \\ \end{bmatrix}$$ Under What conditions is the generalized Vandermonde Matrix Invertible? It is clear that if one lets $x_i=x_j$,then the determinant is Zero, and therefore the principle Vandermonde Determinant $V_p=\prod_{1 \leq i < j\leq n}^n(x_i-x_j)$ can be factored from the $det(V)$. What remains is the schur function, which is a homogeneous polynomial of degree d which depends on $n$, which symmetric over its variables $x_i,1\leq i\leq n$. There is a proof of the fact that the coefficients of the schur function are positive integers which was given by Mitchel long ago.

When one looks at the simplest case where $x_i \in \mathbb{C}, 1\leq i \leq n$ and $n=2$, $$V= \begin{bmatrix} 1 & 1\\ x_1^n & x_2^n\\ \end{bmatrix}$$ one sees that the Vandermonde determinant is invertible when $\frac{x_1}{x_2}$ is not an $n^{th}$ root of unity.

For $n=3$ things become difficult already as $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1}\\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} \end{bmatrix}$$ $det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$.

As far as I see (which is not very far), characterizing the zero set of the this polynomial is not at all simple in either $\mathbb{C^3}$ or $\mathbb{R^3}$ with exception of the trivial case we already mentioned where $x_i=x_j$ for some $i\neq j$ and perhaps some roots of unity. One can guess that over $\mathbb{C^n}$, if the $|x_i|$ are distinct and have no-non trivial relations, then the schur function is non-zero. Which brings me back to my question:

What are the conditions under which the generalized Vandermonde matrix is invertible over $\mathbb{R^n}$ or $\mathbb{C^n}$? Is there Hope?

Well yeah, a little bit.There are some results one could see from Mitchel's result about the positivity of the coefficients of the Schur function. If we are working over $\mathbb{R^n}$, then as a consequence of the positivity of the coefficients mentioned by Mitchel, one can deduce that if $x_i$ are distinct and positive for $1 \leq i \leq n$, then the schur function takes on positive values and therefore non-zero. This result is mentioned in this Paper.

Examples:

1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1 & x_2 & x_3\\ x_1^{3} & x_2^{3} & x_3^{3} \end{bmatrix}$$ where $m_1 = 1, m_2 = 3$, then $det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$, so our schur function is the elementary symmetric function $x_1+x_2+x_3$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

  1. 1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{4} & x_2^{4} & x_3^{4}\\ x_1^{6} & x_2^{6} & x_3^{6} \end{bmatrix}$$ where $m_1 = 4, m_2 = 6$, then

$det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$

$det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$, so our schur function is $ (x_1+x_2)(x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

  1. 1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{4} & x_2^{4} & x_3^{4}\\ x_1^{7} & x_2^{7} & x_3^{7} \end{bmatrix}$$ where $m_1 = 4, m_2 = 7$, then

$det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$ *

$(x_2^3 x_1^5+x_3^3 x_1^5+x_2 x_3^2 x_1^5+x_2^2 x_3 x_1^5+x_2^4 x_1^4+x_3^4 x_1^4+$ $2 x_2 x_3^3 x_1^4+2 x_2^2 x_3^2 x_1^4+2 x_2^3 x_3 x_1^4+x_2^5 x_1^3+x_3^5 x_1^3+$ $2 x_2 x_3^4 x_1^3+3 x_2^2 x_3^3 x_1^3+3 x_2^3 x_3^2 x_1^3+2 x_2^4 x_3 x_1^3+x_2 x_3^5$ $x_1^2+2 x_2^2 x_3^4 x_1^2+3 x_2^3 x_3^3 x_1^2+2 x_2^4 x_3^2 x_1^2+x_2^5 x_3 x_1^2+$ $x_2^2 x_3^5 x_1+2 x_2^3 x_3^4 x_1+2 x_2^4 x_3^3 x_1+x_2^5 x_3^2 x_1+x_2^3 x_3^5+$ $x_2^4 x_3^4+x_2^5 x_3^3)$

So our schur function is everything that we have after dividing out by $V_p$. This function is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

My goal is to really answer the following interesting question, Let ${m_i}$, $i \in \mathbb{N}$ be an increasing sequence of distinct positive integers, then consider the schur function obtained by dividing out the following Vandermonde Determinant by the principle Vandermonde:

$$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_{i_1}} & x_2^{m_{i_1}} & x_3^{m_{i_1}} & \ldots & x_{n+1}^{m_{i_1}} \\ x_1^{m_{i_2}} & x_2^{m_{i_2}} & x_3^{m_{i_2}} & \ldots & x_{n+1}^{m_{i_2}} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_{i_n}} & x_2^{m_{i_n}} & x_3^{m_{i_n}} & \ldots & x_{n+1}^{m_{i_n}} \\ \end{bmatrix}$$

where $m_{i_j}$ are terms in the sequence ${m_i}$, $i \in \mathbb{N}$ such that $m_{i_{j_1}}< m_{i_{j_2}}$ implies $j_1 < j_2$. Let $\Gamma$ ={all possible $m_{i_j}, 1 \leq j \leq n $}, then each element of $\Gamma$ corresponds to a unique schur function. Let $S_{\Gamma}$ ={schur functions induced by $\Gamma$}. Now Let $s \in S_{\Gamma}$ and $Z_s$ ={zeros of $s$}, then I would like to show that $\cap_{s \in S_{\Gamma} }Z_s$ is empty.

Let $m_i\in \mathbb{N}, 1\leq i \leq n $ such that wlog if $m_i < m_j\in \mathbb{Z^+}$ then $i < j$. Consider the Vandermonde matrix $$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1} & \ldots & x_{n+1}^{m_1} \\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} & \ldots & x_{n+1}^{m_2} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_n} & x_2^{m_n} & x_3^{m_n} & \ldots & x_{n+1}^{m_n} \\ \end{bmatrix}$$ Under What conditions is the generalized Vandermonde Matrix Invertible? It is clear that if one lets $x_i=x_j$,then the determinant is Zero, and therefore the principle Vandermonde Determinant $V_p=\prod_{1 \leq i < j\leq n}^n(x_i-x_j)$ can be factored from the $det(V)$. What remains is the schur function, which is a homogeneous polynomial of degree d which depends on $n$, which symmetric over its variables $x_i,1\leq i\leq n$. There is a proof of the fact that the coefficients of the schur function are positive integers which was given by Mitchel long ago.

When one looks at the simplest case where $x_i \in \mathbb{C}, 1\leq i \leq n$ and $n=2$, $$V= \begin{bmatrix} 1 & 1\\ x_1^n & x_2^n\\ \end{bmatrix}$$ one sees that the Vandermonde determinant is invertible when $\frac{x_1}{x_2}$ is not an $n^{th}$ root of unity.

For $n=3$ things become difficult already as $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{m_1} & x_2^{m_1} & x_3^{m_1}\\ x_1^{m_2} & x_2^{m_2} & x_3^{m_2} \end{bmatrix}$$ $det(V_3) = x_2^{m_2} x_1^{m_1}-x_3^{m_2} x_1^{m_1}-x_2^{m_1} x_1^{m_2}+x_3^{m_1} x_1^{m_2}-x_2^{m_2} x_3^{m_1}+x_2^{m_1} x_3^{m_2}$.

As far as I see (which is not very far), characterizing the zero set of the this polynomial is not at all simple in either $\mathbb{C^3}$ or $\mathbb{R^3}$ with exception of the trivial case we already mentioned where $x_i=x_j$ for some $i\neq j$ and perhaps some roots of unity. One can guess that over $\mathbb{C^n}$, if the $|x_i|$ are distinct and have no-non trivial relations, then the schur function is non-zero. Which brings me back to my question:

What are the conditions under which the generalized Vandermonde matrix is invertible over $\mathbb{R^n}$ or $\mathbb{C^n}$? Is there Hope?

Well yeah, a little bit.There are some results one could see from Mitchel's result about the positivity of the coefficients of the Schur function. If we are working over $\mathbb{R^n}$, then as a consequence of the positivity of the coefficients mentioned by Mitchel, one can deduce that if $x_i$ are distinct and positive for $1 \leq i \leq n$, then the schur function takes on positive values and therefore non-zero. This result is mentioned in this Paper.

Examples:

1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1 & x_2 & x_3\\ x_1^{3} & x_2^{3} & x_3^{3} \end{bmatrix}$$ where $m_1 = 1, m_2 = 3$, then $det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3) (x_1+x_2+x_3)$, so our schur function is the elementary symmetric function $x_1+x_2+x_3$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

  1. 1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{4} & x_2^{4} & x_3^{4}\\ x_1^{6} & x_2^{6} & x_3^{6} \end{bmatrix}$$ where $m_1 = 4, m_2 = 6$, then

$det(V)=-x_2^4 x_1^6+x_3^4 x_1^6+x_2^6 x_1^4-x_3^6 x_1^4+x_2^4 x_3^6-x_2^6 x_3^4$

$det(V)=-(x_1-x_2) (x_1+x_2) (x_1-x_3) (x_2-x_3) (x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$, so our schur function is $ (x_1+x_2)(x_1+x_3) (x_2+x_3) (x_1^2 x_2^2+x_3^2 x_2^2+x_1^2 x_3^2)$ which is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

  1. 1)Let $V_3$ be a the following $3$x$3$ matrix: $$V_3= \begin{bmatrix} 1 & 1 & 1\\ x_1^{4} & x_2^{4} & x_3^{4}\\ x_1^{7} & x_2^{7} & x_3^{7} \end{bmatrix}$$ where $m_1 = 4, m_2 = 7$, then

$det(V)=-(x_1-x_2) (x_1-x_3) (x_2-x_3)$ *

$(x_2^3 x_1^5+x_3^3 x_1^5+x_2 x_3^2 x_1^5+x_2^2 x_3 x_1^5+x_2^4 x_1^4+x_3^4 x_1^4+$ $2 x_2 x_3^3 x_1^4+2 x_2^2 x_3^2 x_1^4+2 x_2^3 x_3 x_1^4+x_2^5 x_1^3+x_3^5 x_1^3+$ $2 x_2 x_3^4 x_1^3+3 x_2^2 x_3^3 x_1^3+3 x_2^3 x_3^2 x_1^3+2 x_2^4 x_3 x_1^3+x_2 x_3^5$ $x_1^2+2 x_2^2 x_3^4 x_1^2+3 x_2^3 x_3^3 x_1^2+2 x_2^4 x_3^2 x_1^2+x_2^5 x_3 x_1^2+$ $x_2^2 x_3^5 x_1+2 x_2^3 x_3^4 x_1+2 x_2^4 x_3^3 x_1+x_2^5 x_3^2 x_1+x_2^3 x_3^5+$ $x_2^4 x_3^4+x_2^5 x_3^3)$

So our schur function is everything that we have after dividing out by $V_p$. This function is non-zero if $x_j \neq x_i \in \mathbb{R^+}$ $\forall i,j$

My goal is to really answer the following interesting question, Let ${m_i}$, $i \in \mathbb{N}$ be an increasing sequence of distinct positive integers, then consider the schur function obtained by dividing out the following Vandermonde Determinant by the principle Vandermonde:

$$V= \begin{bmatrix} 1 & 1 & 1 & \ldots & 1 \\ x_1^{m_{i_1}} & x_2^{m_{i_1}} & x_3^{m_{i_1}} & \ldots & x_{n+1}^{m_{i_1}} \\ x_1^{m_{i_2}} & x_2^{m_{i_2}} & x_3^{m_{i_2}} & \ldots & x_{n+1}^{m_{i_2}} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_1^{m_{i_n}} & x_2^{m_{i_n}} & x_3^{m_{i_n}} & \ldots & x_{n+1}^{m_{i_n}} \\ \end{bmatrix}$$

where $m_{i_j}$ are terms in the sequence ${m_i}$, $i \in \mathbb{N}$ such that $m_{i_{j_1}}< m_{i_{j_2}}$ implies $j_1 < j_2$. Let $\Gamma$ ={all possible $m_{i_j}, 1 \leq j \leq n $}, then each element of $\Gamma$ corresponds to a unique schur function. Let $S_{\Gamma}$ ={schur functions induced by $\Gamma$}. Now Let $s \in S_{\Gamma}$ and $Z_s$ ={zeros of $s$}, then I would like to show that $\cap_{s \in S_{\Gamma} }Z_s$ is empty, provided that there are no non-trivial relations between the variables of the schur function, i.e. the $n$ variables of the schur function form a free abelian group of rank $n$, i.e. if the product of the coordinates of a zero of the schur function does not satisfy that condition then you should exclude it.

added 710 characters in body
Source Link
Loading
added 19 characters in body
Source Link
Loading
added 12 characters in body; deleted 5 characters in body
Source Link
Loading
added 347 characters in body
Source Link
Loading
added 16 characters in body
Source Link
Loading
deleted 536 characters in body; added 2 characters in body
Source Link
Loading
added 1363 characters in body
Source Link
Loading
deleted 4 characters in body; deleted 4 characters in body
Source Link
Loading
added 1131 characters in body; added 20 characters in body; edited body; deleted 4 characters in body; Post Made Community Wiki
Source Link
Loading
added 596 characters in body; deleted 3 characters in body
Source Link
Loading
added 1215 characters in body; added 4 characters in body; added 10 characters in body
Source Link
Loading
added 1 characters in body; edited title
Source Link
Loading
edited body; edited title; edited body
Source Link
Loading
added 337 characters in body
Source Link
Loading
deleted 1 characters in body; edited title; added 105 characters in body; added 1 characters in body
Source Link
Loading
Source Link
Loading