#12091: chained inequalities bug in add_constraint to MixedIntegerLinearProgram
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Reporter: dimpase | Owner: ncohen
Type: defect | Status: new
Priority: major | Milestone: sage-5.4
Component: linear programming | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Changes (by dimpase):
* cc: vbraun (added)
* priority: critical => major
Old description:
> constraints with 0 on one size of an inequality get lost, somehow:
> {{{
> sage: p = MixedIntegerLinearProgram(maximization=True)
> sage: A = p.new_variable()
> sage: p.add_constraint(A[0]==1)
> sage: p.add_constraint(0<= A[1]-A[0])
> sage: p.show()
> Maximization:
>
> Constraints:
> 1.0 <= x_0 <= 1.0
> Variables:
> x_0 is a continuous variable (min=0.0, max=+oo)
> x_1 is a continuous variable (min=0.0, max=+oo)
> }}}
>
> homogenizing the constraint fixes this problem:
> {{{
> sage: p.add_constraint(0*A[0]<= A[1]-A[0])
> sage: p.show()
> Maximization:
>
> Constraints:
> 1.0 <= x_0 <= 1.0
> x_0 -x_1 <= 0.0
> Variables:
> x_0 is a continuous variable (min=0.0, max=+oo)
> x_1 is a continuous variable (min=0.0, max=+oo)
> }}}
>
> The bug is in `sage.numerical.mip.LinearConstraint` and
> `sage.numerical.mip.LinearFunction`. More examples:
> {{{
> sage: p = MixedIntegerLinearProgram()
> sage: b = p.new_variable()
> sage: b[0] <= b[1] <= 2 # This is not ok
> x_1 <= 2
> sage: (b[0] <= b[1] <= 2).constraints # Not ok
> [x_1, 2]
> sage: b[0] >= b[1] # This is ok
> x_1 <= x_0
> sage: 1 >= b[1] # This is not since it starts with a constant
> False
> sage: 1 >= b[1] >= 2*b[0] # Not ok
> False
> sage: b[2] >= b[1] >= 2*b[0] # Not ok
> 2 x_0 <= x_1
> }}}
New description:
This ticket was originally started to address problems now dealt with in
#13646.
Still, the following (chained inequalities don't work) needs to be fixed:
{{{
sage: p = MixedIntegerLinearProgram()
sage: b = p.new_variable()
sage: b[0] <= b[1] <= 2 # This is not ok
x_1 <= 2
sage: (b[0] <= b[1] <= 2).constraints # Not ok
[x_1, 2]
sage: 1 >= b[1] >= 2*b[0] # Not ok (note that without #13646 this returns
False)
2 x_0 <= x_1
sage: b[2] >= b[1] >= 2*b[0] # Not ok
2 x_0 <= x_1
}}}
See also this
[http://permalink.gmane.org/gmane.comp.mathematics.sage.support/29948
sage-support thread].
--
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12091#comment:25>
Sage <http://www.sagemath.org>
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