Having read the link:

Why maximum/minimum of linear programming occurs at a vertex?

I understand why the optimal solution of any linear programming problem must be on the corner or lies on a face of a convex polygon. But my question is about a proof given below:


I do not know why the next argument was

$$x^\ast - \frac{\epsilon}{2} \frac{c}{||c||} \in P$$

Sorry to ask this question, but I can't imagine where the term

$$\frac{\epsilon}{2} \frac{c}{||c||}$$

is coming from. Anyone, please enlighten me. Thanks.

Isn't it the contradiction will still be true even if we have, $$x^\ast - \frac{c}{||c||}$$.

The problem may be this construction may not be in $P$. Now, I really want to visualize the expressions $\frac{\epsilon}{2} \frac{c}{||c||} $ and $\frac{c}{||c||}$. Thanks.


In the proof, the assumption that $x^\ast$ is in the interior of $P$ is shown to lead to a contradiction.


$$z = x^\ast - \frac{\epsilon}{2} \frac{c}{||c||}$$


$$||z - x^\ast|| = \frac{\epsilon}{2} \frac{||c||}{||c||}= \frac{\epsilon}{2} < \epsilon,$$


$$z \in B_{\epsilon}(x^\ast) \subset P.$$

This was chosen to obtain a feasible point $z \in P$, such that $c^Tz < c^Tx^\ast$

Since $x^\ast$ is optimal we have a contradiction, implying that $x^\ast$ cannot lie in the interior of $P$.

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  • $\begingroup$ thanks for the explanation, but can I clarify what is the difference between c and ||c|| ? is that the modulus? and $z$ must be in the interior of the ball centered at x* with radius $\epsilon$? ( I think I somehow get it) $\endgroup$ – Keneth Adrian Nov 2 '14 at 7:19
  • $\begingroup$ @KenethAdrian: Yes, $||c||$ is the norm (magnitude) of the vector c. With that choice we get a point in the ball where the objective function is smaller than the minimum at $x^{\ast}$, a contradiction. $\endgroup$ – RRL Nov 2 '14 at 7:31
  • 2
    $\begingroup$ Yes, $\|c\|$ is the norm (or length) of the vector $c$. So $\frac{c}{\|c\|}$ as just a fancy way of saying "a unit vector in the direction of $c$". $\endgroup$ – bubba Nov 2 '14 at 7:39
  • $\begingroup$ so really instead of $\frac{\epsilon}{2}$, they could chosen $\alpha \epsilon$ as long as $\alpha < 1$. $\endgroup$ – Kiran K. Feb 7 '16 at 16:47
  • $\begingroup$ @Kiran K: Yes you just need to get a contradiction by producing a point that is both in the interior of $P$ and such that $c^Tz < c^Tx^*$. $\endgroup$ – RRL Feb 7 '16 at 23:01

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