Linear Algebra - Positive-Definiteness in Vectors?












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I was reading up on the inner product over at this Wikipedia page, and I noticed, in the given definition, the use of the term "positive-definiteness". Now, from what I know, this is terminology one would describe a quadratic form with; more specifically, given $Q(vec x) = vec{x}:^TAvec{x}$ and its diagonalised form $Q(vec x) = vec{y}:^TDvec{y}$ with $D$ containing the eigenvalues of $A$ and $vec y$ expressed in the eigenbasis of $A$, one would say that $Q(vec x)$ is positive-definite when the entries on the diagonal of $D$ are strictly positive.



After the use of said term on the linked page, it elaborates that positive-definiteness apparently means $<vec x,vec x>:geq0$, but I have a hard time figuring out what this has to do with quadratic forms, and certainly why the greater-than-or-equal-to sign is used directly after a term describing strict positiveness. It might have something to do with a concept called "bilinear forms", but I haven't been taught about them as of yet.










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  • 1




    $begingroup$
    $langle x,xrangle=0$ only for $x=0$
    $endgroup$
    – Shubham Johri
    Jan 9 at 10:58










  • $begingroup$
    an inner product in $Bbb R^n$ can be defined using a positive definite operator $A$ as $$langle x,yrangle_A:=langle Ax,yrangle$$ where the inner product in the RHS is the standard euclidean product. This is used in submanifolds in $Bbb R^n$ to define a local inner product. What is positive definite is the inner product, not the vectors.
    $endgroup$
    – Masacroso
    Jan 9 at 11:17


















0












$begingroup$


I was reading up on the inner product over at this Wikipedia page, and I noticed, in the given definition, the use of the term "positive-definiteness". Now, from what I know, this is terminology one would describe a quadratic form with; more specifically, given $Q(vec x) = vec{x}:^TAvec{x}$ and its diagonalised form $Q(vec x) = vec{y}:^TDvec{y}$ with $D$ containing the eigenvalues of $A$ and $vec y$ expressed in the eigenbasis of $A$, one would say that $Q(vec x)$ is positive-definite when the entries on the diagonal of $D$ are strictly positive.



After the use of said term on the linked page, it elaborates that positive-definiteness apparently means $<vec x,vec x>:geq0$, but I have a hard time figuring out what this has to do with quadratic forms, and certainly why the greater-than-or-equal-to sign is used directly after a term describing strict positiveness. It might have something to do with a concept called "bilinear forms", but I haven't been taught about them as of yet.










share|cite|improve this question









$endgroup$








  • 1




    $begingroup$
    $langle x,xrangle=0$ only for $x=0$
    $endgroup$
    – Shubham Johri
    Jan 9 at 10:58










  • $begingroup$
    an inner product in $Bbb R^n$ can be defined using a positive definite operator $A$ as $$langle x,yrangle_A:=langle Ax,yrangle$$ where the inner product in the RHS is the standard euclidean product. This is used in submanifolds in $Bbb R^n$ to define a local inner product. What is positive definite is the inner product, not the vectors.
    $endgroup$
    – Masacroso
    Jan 9 at 11:17
















0












0








0





$begingroup$


I was reading up on the inner product over at this Wikipedia page, and I noticed, in the given definition, the use of the term "positive-definiteness". Now, from what I know, this is terminology one would describe a quadratic form with; more specifically, given $Q(vec x) = vec{x}:^TAvec{x}$ and its diagonalised form $Q(vec x) = vec{y}:^TDvec{y}$ with $D$ containing the eigenvalues of $A$ and $vec y$ expressed in the eigenbasis of $A$, one would say that $Q(vec x)$ is positive-definite when the entries on the diagonal of $D$ are strictly positive.



After the use of said term on the linked page, it elaborates that positive-definiteness apparently means $<vec x,vec x>:geq0$, but I have a hard time figuring out what this has to do with quadratic forms, and certainly why the greater-than-or-equal-to sign is used directly after a term describing strict positiveness. It might have something to do with a concept called "bilinear forms", but I haven't been taught about them as of yet.










share|cite|improve this question









$endgroup$




I was reading up on the inner product over at this Wikipedia page, and I noticed, in the given definition, the use of the term "positive-definiteness". Now, from what I know, this is terminology one would describe a quadratic form with; more specifically, given $Q(vec x) = vec{x}:^TAvec{x}$ and its diagonalised form $Q(vec x) = vec{y}:^TDvec{y}$ with $D$ containing the eigenvalues of $A$ and $vec y$ expressed in the eigenbasis of $A$, one would say that $Q(vec x)$ is positive-definite when the entries on the diagonal of $D$ are strictly positive.



After the use of said term on the linked page, it elaborates that positive-definiteness apparently means $<vec x,vec x>:geq0$, but I have a hard time figuring out what this has to do with quadratic forms, and certainly why the greater-than-or-equal-to sign is used directly after a term describing strict positiveness. It might have something to do with a concept called "bilinear forms", but I haven't been taught about them as of yet.







linear-algebra vector-spaces inner-product-space quadratic-forms positive-definite






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asked Jan 9 at 10:55









MewMew

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  • 1




    $begingroup$
    $langle x,xrangle=0$ only for $x=0$
    $endgroup$
    – Shubham Johri
    Jan 9 at 10:58










  • $begingroup$
    an inner product in $Bbb R^n$ can be defined using a positive definite operator $A$ as $$langle x,yrangle_A:=langle Ax,yrangle$$ where the inner product in the RHS is the standard euclidean product. This is used in submanifolds in $Bbb R^n$ to define a local inner product. What is positive definite is the inner product, not the vectors.
    $endgroup$
    – Masacroso
    Jan 9 at 11:17
















  • 1




    $begingroup$
    $langle x,xrangle=0$ only for $x=0$
    $endgroup$
    – Shubham Johri
    Jan 9 at 10:58










  • $begingroup$
    an inner product in $Bbb R^n$ can be defined using a positive definite operator $A$ as $$langle x,yrangle_A:=langle Ax,yrangle$$ where the inner product in the RHS is the standard euclidean product. This is used in submanifolds in $Bbb R^n$ to define a local inner product. What is positive definite is the inner product, not the vectors.
    $endgroup$
    – Masacroso
    Jan 9 at 11:17










1




1




$begingroup$
$langle x,xrangle=0$ only for $x=0$
$endgroup$
– Shubham Johri
Jan 9 at 10:58




$begingroup$
$langle x,xrangle=0$ only for $x=0$
$endgroup$
– Shubham Johri
Jan 9 at 10:58












$begingroup$
an inner product in $Bbb R^n$ can be defined using a positive definite operator $A$ as $$langle x,yrangle_A:=langle Ax,yrangle$$ where the inner product in the RHS is the standard euclidean product. This is used in submanifolds in $Bbb R^n$ to define a local inner product. What is positive definite is the inner product, not the vectors.
$endgroup$
– Masacroso
Jan 9 at 11:17






$begingroup$
an inner product in $Bbb R^n$ can be defined using a positive definite operator $A$ as $$langle x,yrangle_A:=langle Ax,yrangle$$ where the inner product in the RHS is the standard euclidean product. This is used in submanifolds in $Bbb R^n$ to define a local inner product. What is positive definite is the inner product, not the vectors.
$endgroup$
– Masacroso
Jan 9 at 11:17












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