DOcplex：对偶的对偶与原始的不一样？

Question

I have a very simple primal problem defined like this:

mdl = Model(name='tubs_primal')

aqua = mdl.continuous_var(name='aqua')
hydro = mdl.continuous_var(name='hydro')
typhoon = mdl.continuous_var(name='typhoon')

pump = mdl.add_constraint(aqua + hydro + typhoon <= 200, 'pump')
hour = mdl.add_constraint(9*aqua + 6*hydro + 8*typhoon<= 1566, 'hour')
tubing = mdl.add_constraint(12*aqua+16*hydro + 13*typhoon <= 2880, 'tubing')

profit = 350*aqua + 300*hydro + 320*typhoon

mdl.maximize(profit)

I also wrote down its dual problem like this

mdl = Model(name='tubs_dual')

pump = mdl.continuous_var(name='pump', lb=None)    
hour = mdl.continuous_var(name='hour', lb=None)    
tubing = mdl.continuous_var(name='tubing', lb=None)

aqua = mdl.add_constraint(pump + 9*hour + 12*tubing >= 350, 'aqua')
hydro = mdl.add_constraint(pump + 6*hour + 16*tubing >= 300, 'hydro')
typhoon = mdl.add_constraint(pump + 8*hour + 13*tubing >= 320, 'typhoon')

cost = 200*pump + 1566*hour + 2880*tubing

mdl.minimize(cost)

The primal code works perfect. However, when I ran the dual problem, CPLEX tells me that the dual objective and primal objective are different. So the dual problem of a dual is not same as primal? I am quite puzzled. Any help? Thanks.

Iteration log . . .
Iteration:     1   Dual objective     =         60900.000000
objective: 66100.000
  pump=200.000
  hour=16.667

Answer 1

The reason is because of not full log display. Suppose you solve the 2 models primal and dual with primal method

from docplex.mp.model import Model

mdl = Model(name='tubs_primal')

aqua = mdl.continuous_var(name='aqua')
hydro = mdl.continuous_var(name='hydro')
typhoon = mdl.continuous_var(name='typhoon')

pump = mdl.add_constraint(aqua + hydro + typhoon <= 200, 'pump')
hour = mdl.add_constraint(9*aqua + 6*hydro + 8*typhoon<= 1566, 'hour')
tubing = mdl.add_constraint(12*aqua+16*hydro + 13*typhoon <= 2880, 'tubing')

profit = 350*aqua + 300*hydro + 320*typhoon
mdl.maximize(profit)

mdl.parameters.lpmethod=1

mdl.solve(log_output=True)

print("obj=",mdl.objective_value)

mdl2 = Model(name='tubs_dual')

pump = mdl2.continuous_var(name='pump', lb=None)    
hour = mdl2.continuous_var(name='hour', lb=None)    
tubing = mdl2.continuous_var(name='tubing', lb=None)

aqua = mdl2.add_constraint(pump + 9*hour + 12*tubing >= 350, 'aqua')
hydro = mdl2.add_constraint(pump + 6*hour + 16*tubing >= 300, 'hydro')
typhoon = mdl2.add_constraint(pump + 8*hour + 13*tubing >= 320, 'typhoon')

cost = 200*pump + 1566*hour + 2880*tubing

mdl2.minimize(cost)
mdl2.parameters.lpmethod=1

mdl2.solve(log_output=True)

print("obj=",mdl2.objective_value)

Then you will get

Iteration log . . .
Iteration:     1    Objective     =         60900.000000
obj= 66100.0


Iteration log . . .
Iteration:     1    Scaled infeas =             0.000000
Switched to devex.
Iteration:     2    Objective     =         66100.000000
obj= 66100.0