Hello,
I think there might be a bug in how write_lp handles the objective
function. Consider the following simple ILP formulation for signed
domination
===
def signed(G):
I = MixedIntegerLinearProgram(maximization=false)
x = I.new_variable(binary=True)
for u in G:
I.add_constraint(I.sum(2*x[v]-1 for v in (G[u]+[u])) >=1 )
I.set_objective(I.sum(2*x[u]-1 for u in G))
return I
===
The respective formulation for the Petersen graph gives the following
objective function,
===
G = graphs.PetersenGraph()
I = signed(G)
I.show()
Minimization:
2.0 x_0 + 2.0 x_1 + 2.0 x_2 + 2.0 x_3 + 2.0 x_4 + 2.0 x_5 + 2.0 x_6 + 2.0 x_7 +
2.0 x_8 + 2.0 x_9 - 10.0
.................
===
which is fine. Now if we write the LP problem to a file the subtracted constant
is missed:
===
I.write_lp('foo')
$ cat foo.lp
Minimize
obj: 2 x0 + 2 x1 + 2 x2 + 2 x3 + 2 x4 + 2 x5 + 2 x6 + 2 x7 + 2 x8 + 2 x9
.........
===
Hence by solving this LP in Sage or with a *.lp solver, one obtains different
results. Am I missing something or is this a bug in write_lp?
Best,
Jernej
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