I have a network:

enter image description here How do I figure out the maximum and minimum possible flow through each undefined branch?

  • $\begingroup$ Do you know how to distinguish if the flow labeled on a graph is a valid flow, given the capacities? $\endgroup$ – DanielV Feb 18 '16 at 22:02
  • $\begingroup$ i'm assuming as long as all undefined branches are less than the output. so in this case if f1+f2+f3 <= 90? $\endgroup$ – supremus_01 Feb 18 '16 at 22:06
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    $\begingroup$ Define: A = [1 1 0; 0 -1 1; 1 0 1] x = [f1; f2; f3] b = [80; 10; 90] A*x = b f1, f2, f3 >= 0 (i guess) Subject to these equations, LP solve: - minimums: min(f1), min(f2), min(f3) - maximums: max(f1), max(f2), max(f3) $\endgroup$ – agastalver Feb 18 '16 at 22:06
  • $\begingroup$ One thing that needs to be clarified, does the blue text represent flow, or does it represent capacities (with unlabeled capacities being infinite)? $\endgroup$ – DanielV Feb 18 '16 at 22:08
  • $\begingroup$ they're representative of flow @DanielV $\endgroup$ – supremus_01 Feb 18 '16 at 22:11

Flow must meet the following criteria:

  • Flow into a vertex must equal flow out
  • Flow must be non-negative
  • Flow must be less than or equal to capacity for each edge

For this, we'll have to assume the capacities are "$\infty$".

Vertex $A$ limits the flow of $f_1$ to 80, vertex $C$ limits it to 90. So we check if there is a flow through $f_1$ of 80: $f_1 = 80, f_2 = 0, f_3 = 10$ is a valid flow. We can also check if $0$ is a valid minimum: $f_1 = 0, f_2 = 80, f_3 = 90$.

Proceed similarly for $f_2$ and $f_3$. Just check "is there a valid flow with $f_n = x$?", you should be able to guess $x$ from the picture.


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