MHB Why are the quantities equal to 0?

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evinda
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Hello! (Wave)

I am looking at the general form of the Simplex algorithm with the use of tableaux.

$\overline{x_0}$ is a basic non degenarate feasible solution and thus the columns $P_1, \dots, P_m$ are linearly independent.

The first step is to create a $(m+1) \times (n+4)$ matrix as follows:

  • At the first column we write the basic columns: $P-1, \dots, P_m$.
  • At the second column we write the values of the corresponding coefficients of the objective funtion.
  • At the third column we write the initial basic feasible non degenerate solution $\overline{x_0}$.
  • At the next $n$ columns we write the elements of the columns of the matrix $A$.
  • The last column remains empty for now.
  • At the last row we write the value $z_0$ of the objective function that corresponds to the solution $\overline{x_0}$ and also the values of the differences $z_k-c_k, k=1, \dots, n$

Remark

The value of $z_k$ is the dot product of the second and the $(3+k)$-th column.Why does it hold that $z_1-c_1=0, \dots, z_m-c_m=0$ ?

Isn't it $z_1-c_1=c_1 \cdot 1 + c_2 \cdot 0+ \dots + c_m \cdot z_m=c_1$? Or am I wrong? (Thinking)
 
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Also, suppose that we are given the following linear programming problem:

$$\max (5x_1-4x_2) \\ x_1-x_2+x_3=6 \\ 3x_1-2x_2+x_4=24 \\ -2x_1+3x_2+x_5=9 \\ x_1 \geq 0, i=1, \dots, 5$$

The first tableau is this:

$$\begin{matrix}
B & c_B & b & P_1 & P_2 & P_3 & P_4 & P_5 & \theta \\
P_3 & 0 & 6 & 1 & -1 & 1 & 0 & 0 & \\
P_4 & 0 & 24 & 3 & -2 & 0 & 1 & 0 & \\
P_5 & 0 & 9 & -2 & 3 & 0 & 0 & 1 & \\
& z & 0 & -5 & 4 & 0& 0 & 0 &
\end{matrix}$$

How did we deduce that $z_1-c_1=-5$ ?
 
Hey evinda! (Smile)

What are $c_k$ and $z_k$? (Wondering)

The objective function is $z=5x_1-4x_2 \quad\Rightarrow\quad z -5x_1 + 4x_2 = 0$.
The bottom row consists of those coefficients of $x_k$. (Nerd)
 
I like Serena said:
The objective function is $z=5x_1-4x_2 \quad\Rightarrow\quad z -5x_1 + 4x_2 = 0$.
The bottom row consists of those coefficients of $x_k$. (Nerd)

Ah I see... (Nod)

What represents the third column of the bottom row? (Thinking)
 
evinda said:
Ah I see... (Nod)

What represents the third column of the bottom row? (Thinking)

Erm... the third column of the bottom row... that seems to be $-5$.
That's the coefficient of $x_1$ in the objective function, multiplied by $-1$. (Thinking)
 
I like Serena said:
Erm... the third column of the bottom row... (Thinking)

I meant the column of b...
Do we write $0$ at the last row since there is no 4th component of $b$ or is something else meant?
 
evinda said:
I meant the column of b...
Do we write $0$ at the last row since there is no 4th component of $b$ or is something else meant?

Indeed, there is no constant in the function we want to maximize, so the corresponding column $b$ has a $0$ in its position. (Nod)
 
I like Serena said:
Indeed, there is no constant in the function we want to maximize, so the corresponding column $b$ has a $0$ in its position. (Nod)

At the second tableaux, at this position there is the number $30$...
So doesn't this mean anything? (Thinking)
 
evinda said:
At the second tableaux, at this position there is the number $30$...
So doesn't this mean anything? (Thinking)

I don't see a second tableau. :confused:

Either way, if there is a value of $30$ there, it means we have a constant in the function we want to maximize or minimize. (Nerd)
 
  • #10
I like Serena said:
I don't see a second tableau. :confused:

This is the second tableau:

$\begin{matrix}
B & c_B & b & P_1 & P_2 & P_3 & P_4 & P_5 & \theta\\
P_1 & 5 & 6 & 1 & -1 & 1 & 0 & 0 & - \\
P_4 & 0 & 6 & 0 & 1 & -3 & 1 & 0 & \frac{6}{1}\\
P_5 & 0 & 21 & 0 & 1 & 2 & 0 & 1 & \frac{21}{1} \\
& z & 30 & 0 & -1 & 5 & 0 & 0 &
\end{matrix}$
I like Serena said:
Either way, if there is a value of $30$ there, it means we have a constant in the function we want to maximize or minimize. (Nerd)
Could you explain it further to me? (Thinking)
 
  • #11
evinda said:
This is the second tableau:

$\begin{matrix}
B & c_B & b & P_1 & P_2 & P_3 & P_4 & P_5 & \theta\\
P_1 & 5 & 6 & 1 & -1 & 1 & 0 & 0 & - \\
P_4 & 0 & 6 & 0 & 1 & -3 & 1 & 0 & \frac{6}{1}\\
P_5 & 0 & 21 & 0 & 1 & 2 & 0 & 1 & \frac{21}{1} \\
& z & 30 & 0 & -1 & 5 & 0 & 0 &
\end{matrix}$

Could you explain it further to me? (Thinking)

I may be wrong about a plus or minus sign, for which you should check your course material. (Worried)
But I think the bottom row means that we want to maximize $z$ under the constraint that:
$$z - x_2 + 5x_3 = 30 \quad \Rightarrow \quad z = 30 + x_2 - 5x_3$$
So if I'm right about the signs, we want to maximize $30 + x_2 - 5x_3$.
 
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