Linear dependence of three functions: $f(x) = sin(x)$, $g(x) = cos(x)$ and $h(x)=x$
$begingroup$
Let $mathbb{R}^mathbb{R} $ be a vector-space of functions $f:mathbb{R}→mathbb{R}$.
The following functions in the vector space are defined as follows:
$$f(x) = sin(x),qquad g(x) = cos(x),qquad h(x) = x.$$
Is the triple $(f,g,h)$ linearly dependent?
I have a feeling it is, because $h(x)$ is the identity function, so $h(sin(x)) = sin(x)$, and I know the definitions, I can't figure out how to put my explanation (if it is correct) properly.
Thanks in advance.
linear-algebra functions vector-spaces
$endgroup$
add a comment |
$begingroup$
Let $mathbb{R}^mathbb{R} $ be a vector-space of functions $f:mathbb{R}→mathbb{R}$.
The following functions in the vector space are defined as follows:
$$f(x) = sin(x),qquad g(x) = cos(x),qquad h(x) = x.$$
Is the triple $(f,g,h)$ linearly dependent?
I have a feeling it is, because $h(x)$ is the identity function, so $h(sin(x)) = sin(x)$, and I know the definitions, I can't figure out how to put my explanation (if it is correct) properly.
Thanks in advance.
linear-algebra functions vector-spaces
$endgroup$
$begingroup$
Linear dependent means there are coefficients $a,b,c$ (not all equal to zero) such that $asin(x)+bcos(x)+cx=0$ for all $xinmathbb{R}$
$endgroup$
– Fakemistake
Dec 1 '18 at 7:38
$begingroup$
$h(x)$ is not the identity function when the operation is function addition, $f(x) = 0$ is. Function composition does not form a vector space, so linear dependence is as defined in the above comment.
$endgroup$
– Anthony Ter
Dec 1 '18 at 7:40
add a comment |
$begingroup$
Let $mathbb{R}^mathbb{R} $ be a vector-space of functions $f:mathbb{R}→mathbb{R}$.
The following functions in the vector space are defined as follows:
$$f(x) = sin(x),qquad g(x) = cos(x),qquad h(x) = x.$$
Is the triple $(f,g,h)$ linearly dependent?
I have a feeling it is, because $h(x)$ is the identity function, so $h(sin(x)) = sin(x)$, and I know the definitions, I can't figure out how to put my explanation (if it is correct) properly.
Thanks in advance.
linear-algebra functions vector-spaces
$endgroup$
Let $mathbb{R}^mathbb{R} $ be a vector-space of functions $f:mathbb{R}→mathbb{R}$.
The following functions in the vector space are defined as follows:
$$f(x) = sin(x),qquad g(x) = cos(x),qquad h(x) = x.$$
Is the triple $(f,g,h)$ linearly dependent?
I have a feeling it is, because $h(x)$ is the identity function, so $h(sin(x)) = sin(x)$, and I know the definitions, I can't figure out how to put my explanation (if it is correct) properly.
Thanks in advance.
linear-algebra functions vector-spaces
linear-algebra functions vector-spaces
edited Dec 1 '18 at 14:53
Martin Sleziak
44.7k8117272
44.7k8117272
asked Dec 1 '18 at 7:32
TegernakoTegernako
897
897
$begingroup$
Linear dependent means there are coefficients $a,b,c$ (not all equal to zero) such that $asin(x)+bcos(x)+cx=0$ for all $xinmathbb{R}$
$endgroup$
– Fakemistake
Dec 1 '18 at 7:38
$begingroup$
$h(x)$ is not the identity function when the operation is function addition, $f(x) = 0$ is. Function composition does not form a vector space, so linear dependence is as defined in the above comment.
$endgroup$
– Anthony Ter
Dec 1 '18 at 7:40
add a comment |
$begingroup$
Linear dependent means there are coefficients $a,b,c$ (not all equal to zero) such that $asin(x)+bcos(x)+cx=0$ for all $xinmathbb{R}$
$endgroup$
– Fakemistake
Dec 1 '18 at 7:38
$begingroup$
$h(x)$ is not the identity function when the operation is function addition, $f(x) = 0$ is. Function composition does not form a vector space, so linear dependence is as defined in the above comment.
$endgroup$
– Anthony Ter
Dec 1 '18 at 7:40
$begingroup$
Linear dependent means there are coefficients $a,b,c$ (not all equal to zero) such that $asin(x)+bcos(x)+cx=0$ for all $xinmathbb{R}$
$endgroup$
– Fakemistake
Dec 1 '18 at 7:38
$begingroup$
Linear dependent means there are coefficients $a,b,c$ (not all equal to zero) such that $asin(x)+bcos(x)+cx=0$ for all $xinmathbb{R}$
$endgroup$
– Fakemistake
Dec 1 '18 at 7:38
$begingroup$
$h(x)$ is not the identity function when the operation is function addition, $f(x) = 0$ is. Function composition does not form a vector space, so linear dependence is as defined in the above comment.
$endgroup$
– Anthony Ter
Dec 1 '18 at 7:40
$begingroup$
$h(x)$ is not the identity function when the operation is function addition, $f(x) = 0$ is. Function composition does not form a vector space, so linear dependence is as defined in the above comment.
$endgroup$
– Anthony Ter
Dec 1 '18 at 7:40
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Your feeling is not correct. Recall that, by definition, if such functions are linearly dependent then there are real numbers $A,B,C$ not all zero such that
$$Asin(x)+Bcos(x)+Cx=0 quad forall xin mathbb{R}.$$
Now by letting $x=0,pi/2,pi$, we have three linear equations
$$begin{cases}
Acdot 0+Bcdot 1+Ccdot 0=0qquad &(x=0)\
Acdot 1+Bcdot 0+Ccdot frac{pi}{2}=0qquad &(x=frac{pi}{2})\
Acdot 0+Bcdot (-1)+Ccdot pi=0qquad &(x=pi)\
end{cases}$$
What may we conclude about $A,B,C$? Can you take it from here?
$endgroup$
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
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@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
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Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
|
show 1 more comment
$begingroup$
Asserting that ${f,g,h}$ is linearly independent means that$$(forall a,b,cinmathbb{R}):af+bg+ch=0implies a=b=c=0.$$If $af+bg+ch=0$, then $af(0)+bg(0)+ch(0)=0$, which means that $b=0$. You still have $af+ch=0$. But then $afleft(fracpi2right)+cfracpi2=0$, which means that $a+cfracpi2=0$, and $af(pi)+ch(pi)=0$, which means that $cpi=0$. So, $a=c=0$ too.
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Your feeling is not correct. Recall that, by definition, if such functions are linearly dependent then there are real numbers $A,B,C$ not all zero such that
$$Asin(x)+Bcos(x)+Cx=0 quad forall xin mathbb{R}.$$
Now by letting $x=0,pi/2,pi$, we have three linear equations
$$begin{cases}
Acdot 0+Bcdot 1+Ccdot 0=0qquad &(x=0)\
Acdot 1+Bcdot 0+Ccdot frac{pi}{2}=0qquad &(x=frac{pi}{2})\
Acdot 0+Bcdot (-1)+Ccdot pi=0qquad &(x=pi)\
end{cases}$$
What may we conclude about $A,B,C$? Can you take it from here?
$endgroup$
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
$begingroup$
@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
$begingroup$
Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
|
show 1 more comment
$begingroup$
Your feeling is not correct. Recall that, by definition, if such functions are linearly dependent then there are real numbers $A,B,C$ not all zero such that
$$Asin(x)+Bcos(x)+Cx=0 quad forall xin mathbb{R}.$$
Now by letting $x=0,pi/2,pi$, we have three linear equations
$$begin{cases}
Acdot 0+Bcdot 1+Ccdot 0=0qquad &(x=0)\
Acdot 1+Bcdot 0+Ccdot frac{pi}{2}=0qquad &(x=frac{pi}{2})\
Acdot 0+Bcdot (-1)+Ccdot pi=0qquad &(x=pi)\
end{cases}$$
What may we conclude about $A,B,C$? Can you take it from here?
$endgroup$
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
$begingroup$
@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
$begingroup$
Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
|
show 1 more comment
$begingroup$
Your feeling is not correct. Recall that, by definition, if such functions are linearly dependent then there are real numbers $A,B,C$ not all zero such that
$$Asin(x)+Bcos(x)+Cx=0 quad forall xin mathbb{R}.$$
Now by letting $x=0,pi/2,pi$, we have three linear equations
$$begin{cases}
Acdot 0+Bcdot 1+Ccdot 0=0qquad &(x=0)\
Acdot 1+Bcdot 0+Ccdot frac{pi}{2}=0qquad &(x=frac{pi}{2})\
Acdot 0+Bcdot (-1)+Ccdot pi=0qquad &(x=pi)\
end{cases}$$
What may we conclude about $A,B,C$? Can you take it from here?
$endgroup$
Your feeling is not correct. Recall that, by definition, if such functions are linearly dependent then there are real numbers $A,B,C$ not all zero such that
$$Asin(x)+Bcos(x)+Cx=0 quad forall xin mathbb{R}.$$
Now by letting $x=0,pi/2,pi$, we have three linear equations
$$begin{cases}
Acdot 0+Bcdot 1+Ccdot 0=0qquad &(x=0)\
Acdot 1+Bcdot 0+Ccdot frac{pi}{2}=0qquad &(x=frac{pi}{2})\
Acdot 0+Bcdot (-1)+Ccdot pi=0qquad &(x=pi)\
end{cases}$$
What may we conclude about $A,B,C$? Can you take it from here?
edited Dec 1 '18 at 10:44
answered Dec 1 '18 at 7:52
Robert ZRobert Z
94.3k1063134
94.3k1063134
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
$begingroup$
@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
$begingroup$
Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
|
show 1 more comment
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
$begingroup$
@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
$begingroup$
Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
@caverac What do you mean? OP is interested in the vector space $mathbb{R}^mathbb{R}$, so the domain for $x$ is the whole real line.
$endgroup$
– Robert Z
Dec 1 '18 at 10:14
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
$begingroup$
You mean "your feeling is incorrect", right?
$endgroup$
– Pedro A
Dec 1 '18 at 10:26
$begingroup$
@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
$begingroup$
@PedroA Yes, you are right, thanks for pointing out!
$endgroup$
– Robert Z
Dec 1 '18 at 10:45
$begingroup$
Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
Yes, but I realize issue is with my approach
$endgroup$
– caverac
Dec 1 '18 at 12:01
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
$begingroup$
@RobertZ that we ultimately get that A=B=C=0 for all cases?
$endgroup$
– Tegernako
Dec 1 '18 at 12:49
|
show 1 more comment
$begingroup$
Asserting that ${f,g,h}$ is linearly independent means that$$(forall a,b,cinmathbb{R}):af+bg+ch=0implies a=b=c=0.$$If $af+bg+ch=0$, then $af(0)+bg(0)+ch(0)=0$, which means that $b=0$. You still have $af+ch=0$. But then $afleft(fracpi2right)+cfracpi2=0$, which means that $a+cfracpi2=0$, and $af(pi)+ch(pi)=0$, which means that $cpi=0$. So, $a=c=0$ too.
$endgroup$
add a comment |
$begingroup$
Asserting that ${f,g,h}$ is linearly independent means that$$(forall a,b,cinmathbb{R}):af+bg+ch=0implies a=b=c=0.$$If $af+bg+ch=0$, then $af(0)+bg(0)+ch(0)=0$, which means that $b=0$. You still have $af+ch=0$. But then $afleft(fracpi2right)+cfracpi2=0$, which means that $a+cfracpi2=0$, and $af(pi)+ch(pi)=0$, which means that $cpi=0$. So, $a=c=0$ too.
$endgroup$
add a comment |
$begingroup$
Asserting that ${f,g,h}$ is linearly independent means that$$(forall a,b,cinmathbb{R}):af+bg+ch=0implies a=b=c=0.$$If $af+bg+ch=0$, then $af(0)+bg(0)+ch(0)=0$, which means that $b=0$. You still have $af+ch=0$. But then $afleft(fracpi2right)+cfracpi2=0$, which means that $a+cfracpi2=0$, and $af(pi)+ch(pi)=0$, which means that $cpi=0$. So, $a=c=0$ too.
$endgroup$
Asserting that ${f,g,h}$ is linearly independent means that$$(forall a,b,cinmathbb{R}):af+bg+ch=0implies a=b=c=0.$$If $af+bg+ch=0$, then $af(0)+bg(0)+ch(0)=0$, which means that $b=0$. You still have $af+ch=0$. But then $afleft(fracpi2right)+cfracpi2=0$, which means that $a+cfracpi2=0$, and $af(pi)+ch(pi)=0$, which means that $cpi=0$. So, $a=c=0$ too.
answered Dec 1 '18 at 7:56
José Carlos SantosJosé Carlos Santos
154k22123226
154k22123226
add a comment |
add a comment |
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$begingroup$
Linear dependent means there are coefficients $a,b,c$ (not all equal to zero) such that $asin(x)+bcos(x)+cx=0$ for all $xinmathbb{R}$
$endgroup$
– Fakemistake
Dec 1 '18 at 7:38
$begingroup$
$h(x)$ is not the identity function when the operation is function addition, $f(x) = 0$ is. Function composition does not form a vector space, so linear dependence is as defined in the above comment.
$endgroup$
– Anthony Ter
Dec 1 '18 at 7:40