Why are the quantities equal to 0?

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Discussion Overview

The discussion revolves around the Simplex algorithm in linear programming, specifically focusing on the construction and interpretation of tableaux. Participants explore the relationships between the objective function coefficients and the tableau entries, questioning why certain quantities equal zero and how to interpret values in the tableaux.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant describes the structure of the Simplex tableau and questions why $z_k - c_k = 0$ for $k=1, \dots, m$.
  • Another participant provides a linear programming problem and asks how the value $z_1 - c_1 = -5$ is derived.
  • Several participants seek clarification on the meaning of $c_k$ and $z_k$, noting that the bottom row of the tableau contains coefficients of the objective function.
  • There is a discussion about the third column of the bottom row, with one participant suggesting it represents $-5$, the coefficient of $x_1$ in the objective function.
  • Questions arise regarding the last row of the tableau and whether a zero is appropriate due to the absence of a constant term in the objective function.
  • Another participant notes that a value of $30$ appears in the second tableau and questions its significance, suggesting it indicates a constant in the function to be maximized or minimized.
  • One participant attempts to clarify the meaning of the second tableau and expresses uncertainty about the signs in the equations presented.

Areas of Agreement / Disagreement

Participants express uncertainty and seek clarification on various aspects of the Simplex algorithm and tableau interpretation. There is no consensus on the implications of the values in the tableaux or the correctness of the signs in the equations.

Contextual Notes

Participants reference specific tableau structures and values but do not resolve the underlying assumptions or mathematical steps involved in their interpretations.

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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