$(D^5-D)y = 12e^x$ using exponential shift
$begingroup$
The equation is $$(D^5-D)y = 12e^x$$
Here, the general solution of homogenous (D^5-D)y=0 is $y_g = c_1 + c_2 cos x + c_3 sin x + c_4 e^x + c_5 e^{-x}$. Now I can apply two methods:
Assume particular solution to be $Axe^x$ and solve. This is linearly independant from $y_g$. But finding $D^5$ can be hectic.
Use exponential shift:
$$(D^5-D)y = 12e^x\
y = 12 e^xfrac{1}{(D+1)^5-(D+1)}(1) \
= 12 e^xfrac{1}{(D+1)^4-1}(1)\
= 12e^x (1+(D+1)^4 + ....)$$
This gives strange answer. What is the proper method for this?
calculus ordinary-differential-equations
$endgroup$
add a comment |
$begingroup$
The equation is $$(D^5-D)y = 12e^x$$
Here, the general solution of homogenous (D^5-D)y=0 is $y_g = c_1 + c_2 cos x + c_3 sin x + c_4 e^x + c_5 e^{-x}$. Now I can apply two methods:
Assume particular solution to be $Axe^x$ and solve. This is linearly independant from $y_g$. But finding $D^5$ can be hectic.
Use exponential shift:
$$(D^5-D)y = 12e^x\
y = 12 e^xfrac{1}{(D+1)^5-(D+1)}(1) \
= 12 e^xfrac{1}{(D+1)^4-1}(1)\
= 12e^x (1+(D+1)^4 + ....)$$
This gives strange answer. What is the proper method for this?
calculus ordinary-differential-equations
$endgroup$
add a comment |
$begingroup$
The equation is $$(D^5-D)y = 12e^x$$
Here, the general solution of homogenous (D^5-D)y=0 is $y_g = c_1 + c_2 cos x + c_3 sin x + c_4 e^x + c_5 e^{-x}$. Now I can apply two methods:
Assume particular solution to be $Axe^x$ and solve. This is linearly independant from $y_g$. But finding $D^5$ can be hectic.
Use exponential shift:
$$(D^5-D)y = 12e^x\
y = 12 e^xfrac{1}{(D+1)^5-(D+1)}(1) \
= 12 e^xfrac{1}{(D+1)^4-1}(1)\
= 12e^x (1+(D+1)^4 + ....)$$
This gives strange answer. What is the proper method for this?
calculus ordinary-differential-equations
$endgroup$
The equation is $$(D^5-D)y = 12e^x$$
Here, the general solution of homogenous (D^5-D)y=0 is $y_g = c_1 + c_2 cos x + c_3 sin x + c_4 e^x + c_5 e^{-x}$. Now I can apply two methods:
Assume particular solution to be $Axe^x$ and solve. This is linearly independant from $y_g$. But finding $D^5$ can be hectic.
Use exponential shift:
$$(D^5-D)y = 12e^x\
y = 12 e^xfrac{1}{(D+1)^5-(D+1)}(1) \
= 12 e^xfrac{1}{(D+1)^4-1}(1)\
= 12e^x (1+(D+1)^4 + ....)$$
This gives strange answer. What is the proper method for this?
calculus ordinary-differential-equations
calculus ordinary-differential-equations
asked Jan 19 at 6:49
jeeajeea
58515
58515
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
1. More efficient way is as follows. Let us write $T=D^5-D$. For any $rinBbb C$, we have
$$
T[e^{rx}]=frac{d^5}{dx^5}e^{rx}-frac{d}{dx}e^{rx}=(r^5-r)e^{rx}.
$$ Now, differentiate with respect to $r$. Then we get
$$
frac{partial }{partial r}T[e^{rx}]=Tleft[frac{partial }{partial r}e^{rx}right]=T[xe^{rx}]=frac{partial }{partial r}left((r^5-r)e^{rx}right)=(5r^4-1)e^{rx}+(r^5-r)xe^{rx}.
$$ We get by letting $r=1$
$$
T[xe^x]=4e^x.
$$ Hence, $T[3xe^x]=12e^x$ and $3xe^x$ is a particular solution of the equation.
2. In fact, correct formula should be
$$
y=12(D^5-D)^{-1}[e^{x}],
$$ not
$$
y=12e^x (D^5-D)^{-1}[1].
$$
Note that $D^5-D=(D-1)(D^4+D^3+D^2+D)$. Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have
$$
(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x.
$$ Finally, since $(D-1)[xe^x]=e^x$, we have
$$
(D-1)^{-1}[3e^x]=(D-1)^{-1}(D^4+D^3+D^2+D)^{-1}[12e^x]=3xe^x.
$$ This also gives a particular solution $3xe^x$.
$endgroup$
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
1
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
add a comment |
Your Answer
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1 Answer
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$begingroup$
1. More efficient way is as follows. Let us write $T=D^5-D$. For any $rinBbb C$, we have
$$
T[e^{rx}]=frac{d^5}{dx^5}e^{rx}-frac{d}{dx}e^{rx}=(r^5-r)e^{rx}.
$$ Now, differentiate with respect to $r$. Then we get
$$
frac{partial }{partial r}T[e^{rx}]=Tleft[frac{partial }{partial r}e^{rx}right]=T[xe^{rx}]=frac{partial }{partial r}left((r^5-r)e^{rx}right)=(5r^4-1)e^{rx}+(r^5-r)xe^{rx}.
$$ We get by letting $r=1$
$$
T[xe^x]=4e^x.
$$ Hence, $T[3xe^x]=12e^x$ and $3xe^x$ is a particular solution of the equation.
2. In fact, correct formula should be
$$
y=12(D^5-D)^{-1}[e^{x}],
$$ not
$$
y=12e^x (D^5-D)^{-1}[1].
$$
Note that $D^5-D=(D-1)(D^4+D^3+D^2+D)$. Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have
$$
(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x.
$$ Finally, since $(D-1)[xe^x]=e^x$, we have
$$
(D-1)^{-1}[3e^x]=(D-1)^{-1}(D^4+D^3+D^2+D)^{-1}[12e^x]=3xe^x.
$$ This also gives a particular solution $3xe^x$.
$endgroup$
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
1
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
add a comment |
$begingroup$
1. More efficient way is as follows. Let us write $T=D^5-D$. For any $rinBbb C$, we have
$$
T[e^{rx}]=frac{d^5}{dx^5}e^{rx}-frac{d}{dx}e^{rx}=(r^5-r)e^{rx}.
$$ Now, differentiate with respect to $r$. Then we get
$$
frac{partial }{partial r}T[e^{rx}]=Tleft[frac{partial }{partial r}e^{rx}right]=T[xe^{rx}]=frac{partial }{partial r}left((r^5-r)e^{rx}right)=(5r^4-1)e^{rx}+(r^5-r)xe^{rx}.
$$ We get by letting $r=1$
$$
T[xe^x]=4e^x.
$$ Hence, $T[3xe^x]=12e^x$ and $3xe^x$ is a particular solution of the equation.
2. In fact, correct formula should be
$$
y=12(D^5-D)^{-1}[e^{x}],
$$ not
$$
y=12e^x (D^5-D)^{-1}[1].
$$
Note that $D^5-D=(D-1)(D^4+D^3+D^2+D)$. Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have
$$
(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x.
$$ Finally, since $(D-1)[xe^x]=e^x$, we have
$$
(D-1)^{-1}[3e^x]=(D-1)^{-1}(D^4+D^3+D^2+D)^{-1}[12e^x]=3xe^x.
$$ This also gives a particular solution $3xe^x$.
$endgroup$
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
1
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
add a comment |
$begingroup$
1. More efficient way is as follows. Let us write $T=D^5-D$. For any $rinBbb C$, we have
$$
T[e^{rx}]=frac{d^5}{dx^5}e^{rx}-frac{d}{dx}e^{rx}=(r^5-r)e^{rx}.
$$ Now, differentiate with respect to $r$. Then we get
$$
frac{partial }{partial r}T[e^{rx}]=Tleft[frac{partial }{partial r}e^{rx}right]=T[xe^{rx}]=frac{partial }{partial r}left((r^5-r)e^{rx}right)=(5r^4-1)e^{rx}+(r^5-r)xe^{rx}.
$$ We get by letting $r=1$
$$
T[xe^x]=4e^x.
$$ Hence, $T[3xe^x]=12e^x$ and $3xe^x$ is a particular solution of the equation.
2. In fact, correct formula should be
$$
y=12(D^5-D)^{-1}[e^{x}],
$$ not
$$
y=12e^x (D^5-D)^{-1}[1].
$$
Note that $D^5-D=(D-1)(D^4+D^3+D^2+D)$. Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have
$$
(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x.
$$ Finally, since $(D-1)[xe^x]=e^x$, we have
$$
(D-1)^{-1}[3e^x]=(D-1)^{-1}(D^4+D^3+D^2+D)^{-1}[12e^x]=3xe^x.
$$ This also gives a particular solution $3xe^x$.
$endgroup$
1. More efficient way is as follows. Let us write $T=D^5-D$. For any $rinBbb C$, we have
$$
T[e^{rx}]=frac{d^5}{dx^5}e^{rx}-frac{d}{dx}e^{rx}=(r^5-r)e^{rx}.
$$ Now, differentiate with respect to $r$. Then we get
$$
frac{partial }{partial r}T[e^{rx}]=Tleft[frac{partial }{partial r}e^{rx}right]=T[xe^{rx}]=frac{partial }{partial r}left((r^5-r)e^{rx}right)=(5r^4-1)e^{rx}+(r^5-r)xe^{rx}.
$$ We get by letting $r=1$
$$
T[xe^x]=4e^x.
$$ Hence, $T[3xe^x]=12e^x$ and $3xe^x$ is a particular solution of the equation.
2. In fact, correct formula should be
$$
y=12(D^5-D)^{-1}[e^{x}],
$$ not
$$
y=12e^x (D^5-D)^{-1}[1].
$$
Note that $D^5-D=(D-1)(D^4+D^3+D^2+D)$. Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have
$$
(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x.
$$ Finally, since $(D-1)[xe^x]=e^x$, we have
$$
(D-1)^{-1}[3e^x]=(D-1)^{-1}(D^4+D^3+D^2+D)^{-1}[12e^x]=3xe^x.
$$ This also gives a particular solution $3xe^x$.
edited Jan 19 at 10:00
answered Jan 19 at 7:56
SongSong
13.2k632
13.2k632
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
1
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
add a comment |
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
1
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
sorry i dont understand your method 2
$endgroup$
– jeea
Jan 19 at 16:38
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
$$12e^x stackrel{(D^4+D^3+D^2+D)^{-1}}Longrightarrow 3e^xstackrel{(D-1)^{-1}}Longrightarrow 3xe^x.$$ $$12e^x stackrel{D^4+D^3+D^2+D}Longleftarrow 3e^xstackrel{D-1}Longleftarrow 3xe^x.$$Can you please be more specific about which part of the argument you don't understand?
$endgroup$
– Song
Jan 19 at 16:42
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
$begingroup$
Yes the first line only, from 12ex to 3ex
$endgroup$
– jeea
Jan 19 at 17:27
1
1
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
@jeea Since $(D^4+D^3+D^2+D)[e^x]=4e^x$, we have $(D^4+D^3+D^2+D)^{-1}[4e^x]=e^x$. By multiplying $3$, we have $(D^4+D^3+D^2+D)^{-1}[12e^x]=3e^x$.
$endgroup$
– Song
Jan 19 at 17:41
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
$begingroup$
Oh so we have to observe and predict very carefullly
$endgroup$
– jeea
Jan 20 at 4:20
add a comment |
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