Partial Fraction Decomposition with Arbitrary Constant in $intfrac{1}{y^4-K^4}dy$.
$begingroup$
I need to find the partial decomposition of a fraction which contains an arbitrary constant. This is the final step, or close to final, on a larger non-linear differential equation problem. I need to integrate...
$$intfrac{1}{y^4-K^4}dy$$
Where $K$ is an arbitrary constant. I'm prettry sure I need to break this fraction up with partial fractions. It's been a long while since I've used partial fractions, and I've tried to brush up, but I can't figure out how to split the fraction up into partials with the unknown constant. I know how I would do it if $K$ where an actual known number, and I've tried just going forward with the way that I know, but I'm getting nowhere. How would I go about splitting...
$$frac{1}{y^4-K^4}$$
into partial fractions so that I can integrate it?
integration partial-fractions
$endgroup$
add a comment |
$begingroup$
I need to find the partial decomposition of a fraction which contains an arbitrary constant. This is the final step, or close to final, on a larger non-linear differential equation problem. I need to integrate...
$$intfrac{1}{y^4-K^4}dy$$
Where $K$ is an arbitrary constant. I'm prettry sure I need to break this fraction up with partial fractions. It's been a long while since I've used partial fractions, and I've tried to brush up, but I can't figure out how to split the fraction up into partials with the unknown constant. I know how I would do it if $K$ where an actual known number, and I've tried just going forward with the way that I know, but I'm getting nowhere. How would I go about splitting...
$$frac{1}{y^4-K^4}$$
into partial fractions so that I can integrate it?
integration partial-fractions
$endgroup$
$begingroup$
Follow the hint from Herr Graubner and proceed as if $K$ were known.
$endgroup$
– Sean Roberson
Apr 15 '18 at 6:02
add a comment |
$begingroup$
I need to find the partial decomposition of a fraction which contains an arbitrary constant. This is the final step, or close to final, on a larger non-linear differential equation problem. I need to integrate...
$$intfrac{1}{y^4-K^4}dy$$
Where $K$ is an arbitrary constant. I'm prettry sure I need to break this fraction up with partial fractions. It's been a long while since I've used partial fractions, and I've tried to brush up, but I can't figure out how to split the fraction up into partials with the unknown constant. I know how I would do it if $K$ where an actual known number, and I've tried just going forward with the way that I know, but I'm getting nowhere. How would I go about splitting...
$$frac{1}{y^4-K^4}$$
into partial fractions so that I can integrate it?
integration partial-fractions
$endgroup$
I need to find the partial decomposition of a fraction which contains an arbitrary constant. This is the final step, or close to final, on a larger non-linear differential equation problem. I need to integrate...
$$intfrac{1}{y^4-K^4}dy$$
Where $K$ is an arbitrary constant. I'm prettry sure I need to break this fraction up with partial fractions. It's been a long while since I've used partial fractions, and I've tried to brush up, but I can't figure out how to split the fraction up into partials with the unknown constant. I know how I would do it if $K$ where an actual known number, and I've tried just going forward with the way that I know, but I'm getting nowhere. How would I go about splitting...
$$frac{1}{y^4-K^4}$$
into partial fractions so that I can integrate it?
integration partial-fractions
integration partial-fractions
edited Aug 7 '18 at 6:50
Nosrati
26.5k62354
26.5k62354
asked Apr 15 '18 at 5:56
Phil FernandezPhil Fernandez
413
413
$begingroup$
Follow the hint from Herr Graubner and proceed as if $K$ were known.
$endgroup$
– Sean Roberson
Apr 15 '18 at 6:02
add a comment |
$begingroup$
Follow the hint from Herr Graubner and proceed as if $K$ were known.
$endgroup$
– Sean Roberson
Apr 15 '18 at 6:02
$begingroup$
Follow the hint from Herr Graubner and proceed as if $K$ were known.
$endgroup$
– Sean Roberson
Apr 15 '18 at 6:02
$begingroup$
Follow the hint from Herr Graubner and proceed as if $K$ were known.
$endgroup$
– Sean Roberson
Apr 15 '18 at 6:02
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Make the ansatz $$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Multiplying by the denominators we obtain
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
from here you will get an equation system to compute the coefficients
$endgroup$
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
add a comment |
$begingroup$
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
To solve Dr. Graubner's equation
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
as stated above, is a lot of linear algebra. Some of that can be avoided.
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Using $x^4-K^4 = (x-K)(x+K)(x^2+K^2)$, we get
$$1 = A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K)$$
Letting $x=K$, you get
begin{align}
1 &= 4AK^3 \
A &= dfrac{1}{4K^3}
end{align}
Letting $x=K$, you get
begin{align}
1 &= -4BK^3 \
B &= -dfrac{1}{4K^3}
end{align}
Letting $x=iK$, you get $1 = -2K^2(D+iCK)$.
Letting $x=-iK$, you get $1 = -2K^2(D-iCK)$.
So $C = 0$ and $D = -dfrac{1}{2K^2}$.
Hence
$$frac{1}{x^4-K^4}=frac{1}{4K^3(x-K)}-frac{1}{4K^3(x+K)}-frac{1}{2K^2(x^2+K^2)}$$
If you don't want to have to resort to complex variables, you can do this after finding the values of $A$ and $B$.
begin{align}
A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{(x+K)(x^2+K^2)}{4K^3}-dfrac{B(x-K)(x^2+K^2)}{4K^3}+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{x^2+K^2}{2K^2}+(Cx+D)(x^2-K^2) &= 1 \
(Cx+D)(x^2-K^2) &= 1 - dfrac{x^2+K^2}{2K^2} \
(Cx+D)(x^2-K^2) &= -dfrac{x^2-K^2}{2K^2} \
Cx+D &= -dfrac{1}{2K^2}
end{align}
$endgroup$
add a comment |
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2 Answers
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$begingroup$
Make the ansatz $$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Multiplying by the denominators we obtain
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
from here you will get an equation system to compute the coefficients
$endgroup$
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
add a comment |
$begingroup$
Make the ansatz $$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Multiplying by the denominators we obtain
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
from here you will get an equation system to compute the coefficients
$endgroup$
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
add a comment |
$begingroup$
Make the ansatz $$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Multiplying by the denominators we obtain
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
from here you will get an equation system to compute the coefficients
$endgroup$
Make the ansatz $$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Multiplying by the denominators we obtain
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
from here you will get an equation system to compute the coefficients
edited Apr 16 '18 at 13:00
answered Apr 15 '18 at 6:01
Dr. Sonnhard GraubnerDr. Sonnhard Graubner
78.4k42867
78.4k42867
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
add a comment |
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
$begingroup$
Should I combine and absorb K into A, B, C, and D during the process of multiplying out? Or should I keep all K as their own entities?
$endgroup$
– Phil Fernandez
Apr 16 '18 at 1:31
add a comment |
$begingroup$
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
To solve Dr. Graubner's equation
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
as stated above, is a lot of linear algebra. Some of that can be avoided.
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Using $x^4-K^4 = (x-K)(x+K)(x^2+K^2)$, we get
$$1 = A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K)$$
Letting $x=K$, you get
begin{align}
1 &= 4AK^3 \
A &= dfrac{1}{4K^3}
end{align}
Letting $x=K$, you get
begin{align}
1 &= -4BK^3 \
B &= -dfrac{1}{4K^3}
end{align}
Letting $x=iK$, you get $1 = -2K^2(D+iCK)$.
Letting $x=-iK$, you get $1 = -2K^2(D-iCK)$.
So $C = 0$ and $D = -dfrac{1}{2K^2}$.
Hence
$$frac{1}{x^4-K^4}=frac{1}{4K^3(x-K)}-frac{1}{4K^3(x+K)}-frac{1}{2K^2(x^2+K^2)}$$
If you don't want to have to resort to complex variables, you can do this after finding the values of $A$ and $B$.
begin{align}
A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{(x+K)(x^2+K^2)}{4K^3}-dfrac{B(x-K)(x^2+K^2)}{4K^3}+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{x^2+K^2}{2K^2}+(Cx+D)(x^2-K^2) &= 1 \
(Cx+D)(x^2-K^2) &= 1 - dfrac{x^2+K^2}{2K^2} \
(Cx+D)(x^2-K^2) &= -dfrac{x^2-K^2}{2K^2} \
Cx+D &= -dfrac{1}{2K^2}
end{align}
$endgroup$
add a comment |
$begingroup$
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
To solve Dr. Graubner's equation
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
as stated above, is a lot of linear algebra. Some of that can be avoided.
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Using $x^4-K^4 = (x-K)(x+K)(x^2+K^2)$, we get
$$1 = A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K)$$
Letting $x=K$, you get
begin{align}
1 &= 4AK^3 \
A &= dfrac{1}{4K^3}
end{align}
Letting $x=K$, you get
begin{align}
1 &= -4BK^3 \
B &= -dfrac{1}{4K^3}
end{align}
Letting $x=iK$, you get $1 = -2K^2(D+iCK)$.
Letting $x=-iK$, you get $1 = -2K^2(D-iCK)$.
So $C = 0$ and $D = -dfrac{1}{2K^2}$.
Hence
$$frac{1}{x^4-K^4}=frac{1}{4K^3(x-K)}-frac{1}{4K^3(x+K)}-frac{1}{2K^2(x^2+K^2)}$$
If you don't want to have to resort to complex variables, you can do this after finding the values of $A$ and $B$.
begin{align}
A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{(x+K)(x^2+K^2)}{4K^3}-dfrac{B(x-K)(x^2+K^2)}{4K^3}+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{x^2+K^2}{2K^2}+(Cx+D)(x^2-K^2) &= 1 \
(Cx+D)(x^2-K^2) &= 1 - dfrac{x^2+K^2}{2K^2} \
(Cx+D)(x^2-K^2) &= -dfrac{x^2-K^2}{2K^2} \
Cx+D &= -dfrac{1}{2K^2}
end{align}
$endgroup$
add a comment |
$begingroup$
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
To solve Dr. Graubner's equation
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
as stated above, is a lot of linear algebra. Some of that can be avoided.
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Using $x^4-K^4 = (x-K)(x+K)(x^2+K^2)$, we get
$$1 = A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K)$$
Letting $x=K$, you get
begin{align}
1 &= 4AK^3 \
A &= dfrac{1}{4K^3}
end{align}
Letting $x=K$, you get
begin{align}
1 &= -4BK^3 \
B &= -dfrac{1}{4K^3}
end{align}
Letting $x=iK$, you get $1 = -2K^2(D+iCK)$.
Letting $x=-iK$, you get $1 = -2K^2(D-iCK)$.
So $C = 0$ and $D = -dfrac{1}{2K^2}$.
Hence
$$frac{1}{x^4-K^4}=frac{1}{4K^3(x-K)}-frac{1}{4K^3(x+K)}-frac{1}{2K^2(x^2+K^2)}$$
If you don't want to have to resort to complex variables, you can do this after finding the values of $A$ and $B$.
begin{align}
A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{(x+K)(x^2+K^2)}{4K^3}-dfrac{B(x-K)(x^2+K^2)}{4K^3}+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{x^2+K^2}{2K^2}+(Cx+D)(x^2-K^2) &= 1 \
(Cx+D)(x^2-K^2) &= 1 - dfrac{x^2+K^2}{2K^2} \
(Cx+D)(x^2-K^2) &= -dfrac{x^2-K^2}{2K^2} \
Cx+D &= -dfrac{1}{2K^2}
end{align}
$endgroup$
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
To solve Dr. Graubner's equation
$$1=x^3(A+B+C)+x^2(AK-BK+D)+x(AK^2+BK^2-CK^2)+AK^3-BK^3-DK^2$$
as stated above, is a lot of linear algebra. Some of that can be avoided.
$$frac{1}{x^4-K^4}=frac{A}{x-K}+frac{B}{x+K}+frac{Cx+D}{x^2+K^2}$$
Using $x^4-K^4 = (x-K)(x+K)(x^2+K^2)$, we get
$$1 = A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K)$$
Letting $x=K$, you get
begin{align}
1 &= 4AK^3 \
A &= dfrac{1}{4K^3}
end{align}
Letting $x=K$, you get
begin{align}
1 &= -4BK^3 \
B &= -dfrac{1}{4K^3}
end{align}
Letting $x=iK$, you get $1 = -2K^2(D+iCK)$.
Letting $x=-iK$, you get $1 = -2K^2(D-iCK)$.
So $C = 0$ and $D = -dfrac{1}{2K^2}$.
Hence
$$frac{1}{x^4-K^4}=frac{1}{4K^3(x-K)}-frac{1}{4K^3(x+K)}-frac{1}{2K^2(x^2+K^2)}$$
If you don't want to have to resort to complex variables, you can do this after finding the values of $A$ and $B$.
begin{align}
A(x+K)(x^2+K^2)+B(x-K)(x^2+K^2)+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{(x+K)(x^2+K^2)}{4K^3}-dfrac{B(x-K)(x^2+K^2)}{4K^3}+(Cx+D)(x-K)(x+K) &= 1 \
dfrac{x^2+K^2}{2K^2}+(Cx+D)(x^2-K^2) &= 1 \
(Cx+D)(x^2-K^2) &= 1 - dfrac{x^2+K^2}{2K^2} \
(Cx+D)(x^2-K^2) &= -dfrac{x^2-K^2}{2K^2} \
Cx+D &= -dfrac{1}{2K^2}
end{align}
answered Jan 2 at 10:36
steven gregorysteven gregory
18.3k32358
18.3k32358
add a comment |
add a comment |
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$begingroup$
Follow the hint from Herr Graubner and proceed as if $K$ were known.
$endgroup$
– Sean Roberson
Apr 15 '18 at 6:02