How to define a lower bound for POW function?

Ongoing

Yuelan Zhu

May 27, 2022 00:03

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Dear community,

I have looked at the recent posts, however, they did not find a solution to my problem.

Here is my current codes:

# continuous variable indicating flow through link ij.
q = m.addVars(mstree[9].edges(),lb = 0, vtype = GRB.CONTINUOUS, name = "FLOW")
qhelper =  m.addVars(mstree[9].edges(), vtype = GRB.CONTINUOUS, name = "qhelper")

# binary variable indicating if at link ij pipe of diameter k is implemented
d = m.addVars(mstree[9].edges(), pipesize, vtype = GRB.BINARY, name = "DIAMETER")

 #q^1.85
 m.addGenConstrPow(q[i,j],qhelper[i,j],1.85)

m.addConstr(loss[i,j] == 
                10.67*qhelper[i,j]/(150**1.825)*gp.quicksum(d[i,j,k]/k**4.8704 for k in pipesize)
               , "Hydro loss")

However, I got such an error.

lower bound of x variable is less than zero for POW function

I think the lower bound of qhelper is already determined by q but it seems something different. I would be happy if anyone can help me.

Thanks.

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 16, 2025 23:36

  This post is more than three years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
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  Jaromił Najman

  • Gurobi Staff

  May 30, 2022 08:33 Edited

  It looks like the lower bound of your \(q\) variables can be slightly negative due to numerical issues in the model. Note that you set the lower bound to \(0\) explicitly, but there can still be tolerances in play.  Could you try setting the lower bound of your \(q\) variables to something slightly positive, e.g., \(10^{-5}\)?

  If the above does not help, could you please write the model you have to a file via the write method

  m.write("myLP.lp")

  and share the file as described in Posting to the Community Forum?

  Best regards, 
  Jaromił

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  Yuelan Zhu

  May 31, 2022 19:26

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  Hello Jaromił,

  Thanks for your help. 

  I set a positive lower bound of q variables but it did not work. 

  Here is my current model and I will be appreciate if you can give further advance.

  myLP.lp

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