The form of the partial fraction decomposition is;
z ( z + 1 ) 3 = A z + 1 + B ( z + 1 ) 2 + C ( z + 1 ) 3 = z ( z + 1 ) 3 = ( z + 1 ) 2 A + ( z + 1 ) B + C ( z + 1 ) 3 ⇒ z = ( z + 1 ) 2 A + ( z + 1 ) B + C \displaystyle
\frac{z}{\left(z + 1\right)^{3}}=\frac{A}{z + 1}+\frac{B}{\left(z + 1\right)^{2}}+\frac{C}{\left(z + 1\right)^{3}}=\frac{z}{\left(z + 1\right)^{3}}=\frac{\left(z + 1\right)^{2} A + \left(z + 1\right) B + C}{\left(z + 1\right)^{3}}\\
\Rightarrow z=\left(z + 1\right)^{2} A + \left(z + 1\right) B + C\\ ( z + 1 ) 3 z = z + 1 A + ( z + 1 ) 2 B + ( z + 1 ) 3 C = ( z + 1 ) 3 z = ( z + 1 ) 3 ( z + 1 ) 2 A + ( z + 1 ) B + C ⇒ z = ( z + 1 ) 2 A + ( z + 1 ) B + C
Expanding the right hand side yields;
z = z 2 A + 2 z A + z B + A + B + C Collecting like terms yields; z = z 2 A + z ( 2 A + B ) + A + B + C Comparing yields the following system of linear equations: { A = 0 2 A + B = 1 A + B + C = 0 Solving the above equations yields: A = 0 , B = 1 , C = − 1 ∴ z ( z + 1 ) 3 = 0 z + 1 + 1 ( z + 1 ) 2 + − 1 ( z + 1 ) 3 = z ( z + 1 ) 3 = 1 ( z + 1 ) 2 − 1 ( z + 1 ) 3 \displaystyle
z=z^{2} A + 2 z A + z B + A + B + C\\
\text{Collecting like terms yields;}\\
z=z^{2} A + z \left(2 A + B\right) + A + B + C\\
\text{Comparing yields the following system of linear equations:}\\
\qquad\qquad\qquad\qquad\qquad\qquad\qquad\begin{cases} A = 0\\2 A + B = 1\\A + B + C = 0 \end{cases}\\
\text{Solving the above equations yields: }A=0,\ B=1,\ C=-1\\
\therefore\frac{z}{\left(z + 1\right)^{3}}=\frac{0}{z + 1}+\frac{1}{\left(z + 1\right)^{2}}+\frac{-1}{\left(z + 1\right)^{3}}=\frac{z}{\left(z + 1\right)^{3}}=\frac{1}{\left(z + 1\right)^{2}}-\frac{1}{\left(z + 1\right)^{3}} z = z 2 A + 2 z A + z B + A + B + C Collecting like terms yields; z = z 2 A + z ( 2 A + B ) + A + B + C Comparing yields the following system of linear equations: ⎩ ⎨ ⎧ A = 0 2 A + B = 1 A + B + C = 0 Solving the above equations yields: A = 0 , B = 1 , C = − 1 ∴ ( z + 1 ) 3 z = z + 1 0 + ( z + 1 ) 2 1 + ( z + 1 ) 3 − 1 = ( z + 1 ) 3 z = ( z + 1 ) 2 1 − ( z + 1 ) 3 1
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