Gurobi says the model is infeasible, yet I can come up with a solution by hand (Python)

Answered

yechan pyeong

July 06, 2023 01:34

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I optimized the model below, but guroby says the model is infeasible.

from gurobipy import *

m = Model('test')
a = m.addVar(name="a", vtype=GRB.INTEGER)
b = m.addVar(name="b", vtype=GRB.INTEGER)
c = m.addVar(name="c", vtype=GRB.INTEGER)

m.addConstr(a >= 0)
m.addConstr(a <= 0)
m.addConstr(a <= 3)
m.addConstr(a-b <= 1)
m.addConstr(a+b <= 5)
m.addConstr(b >= -1)
m.addConstr(b <= 1)
m.addConstr(a-c <= 0)
m.addConstr(a+c <= 0)
m.addConstr(c >= -1)
m.addConstr(c <= 1)
m.addConstr(b+c == -1)

objective = LinExpr(-b)
m.setObjective(objective, GRB.MINIMIZE)
m.setParam(GRB.Param.PoolSearchMode, 2)
m.setParam(GRB.Param.PoolSolutions, 10000)
m.setParam(GRB.Param.PoolGap, 1e-4)
m.optimize()

if m.status != GRB.Status.OPTIMAL:
    print('No Solution')
    m.write("file.lp")
else:
    m.setParam(GRB.Param.SolutionNumber, 0)
    print(m.getVarByName("a").Xn)
    print(m.getVarByName("b").Xn)
    print(m.getVarByName("c").Xn)

I think that the model is feasible if a=0, b=-1, c=0. Why is the model infeasible? Any help is appreciated.

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 13, 2026 01:41

  This post is more than two 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.
• 

  Chung-Kyun Han

  • Gurobi Staff

  July 06, 2023 02:35

  Thank you for reaching us.

  The default lower bound of a variable is zero (please refer to our manual about Model.addVar()).
  If you add variables like as follows, you will get the optimal solution you want.

  a = m.addVar(lb=-GRB.INFINITY, name="a", vtype=GRB.INTEGER)
  b = m.addVar(lb=-GRB.INFINITY, name="b", vtype=GRB.INTEGER)
  c = m.addVar(lb=-GRB.INFINITY, name="c", vtype=GRB.INTEGER)

  Whenever you have any questions, feel free to contact us.

   

  Best regards,
  Chung-Kyun Han

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• 

  yechan pyeong

  July 06, 2023 02:58

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  Thank you very much!

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• 

  Riley Clement

  • Gurobi Staff

  July 06, 2023 02:59

  Hi Yechan,

  You should also take a look at our article How do I determine why my model is infeasible?
  
  With this knowledge, you will run

  m.computeIIS()
  m.write("model.ilp")

  and inspect the file that is written.
  
  - Riley

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