[SciPy-User] Problem using linprog
Montgomery-Smith, Stephen
stephen at missouri.edu
Mon Nov 26 22:34:10 EST 2018
Thank you. That helps a little. What is the definition of singular in
this context?
You definitely understood my problem. The correct answer should have
been 0, as you correctly surmised.
I am trying to find a test to see if a union of half planes captures all
of Euclidean n-space.
On 11/26/18 5:40 PM, Chris F Waigl wrote:
> Hi Stephen,
>
> The problem appears to be singular around the solution. A very quick
> exploration shows me that if you replace your upper bound b by a very
> small epsilon > 0, you get a stable result.
>
> For example:
> b = np.zeros(8) + 0.001
>
> fun: -0.11764011575264395
> message: 'Optimization terminated successfully.'
> nit: 6
> slack: array([0. , 0. , 0.40742577, 0. , 0.40742577, 0. , 0. , 0. , 0.88235988])
> status: 0
> success: True
> x: array([0. , 0. , 0. , 0.0834722 , 0.41811509, 0.11764012])
>
> And for print(np.dot(A, result.x)) I get [ 0.001 0.001 -0.00307426 0.001
> -0.00307426 0.001 0.001 0.001 ]
>
> In the objective function, y_2 = y_4 = -3.0742577 * epsilon, and the
> other 6 values also converge towards zero when epsilon -> 0 .
>
> If I read your problem correctly, your objective function is simply (-1)
> times x_5, the last element of x. The approach above would converge
> towards the trivial solution, x = 0, but your solution above minimizes
> f(x) by maximizing x_5 at 1. If we pick out an x_5, then the problem
> collapses to a new problems to find [x_0 ... x_4] so that A[:, 0:7] *
> [x_0 ... x_4]' < b, where b is (-1) * the last column of your A. But the
> objective function is now indeterminate, so there is nothing to optimize.
>
> HTH,
>
> Chris
>
> On Mon, Nov 26, 2018 at 11:43 AM Montgomery-Smith, Stephen
> <stephen at missouri.edu <mailto:stephen at missouri.edu>> wrote:
>
> I am trying to solve a linear programming problem. The constraint is of
> the form A.x <= 0. But linprog gives an answer that doesn't satisfy the
> constraint.
>
> The attached program gives A.x as
>
> [-2.32109228 2.32017594 4.71436317 3.6433767 -4.26629574 2.32384597
> -1.96166184 -4.96206197]
>
> which definitely doesn't satisfy the constraint. Is this a bug, or some
> subtle floating point error?
>
> Program follows (also as attachment):
>
> from scipy.optimize import linprog
> import numpy as np
>
> A = [[0.5919650431077654, -0.5271408402306996, 0.6096719792636803,
> 1.2379670854947114, 0.2656040423387233, -0.972363043155988],
> [-0.5914974900295467, -0.5266568950860249, 0.6105433925177587,
> 1.258297461476007, -0.285688537323182, 0.9726089241528251],
> [-0.593015674004932, 0.5280764198909397, 0.6078385518701857,
> -1.1964319796886902, -0.2223431679788034, -0.9740888117098865],
> [0.5935986604093653, 0.5285277328950352, 0.6068764832493029,
> -1.1752312553140132, 0.19916734259906424, 0.976063912714949],
> [0.593015674004932, -0.5280764198909397, -0.6078385518701857,
> -1.1964319796886902, -0.2223431679788034, -0.9740888117098865],
> [-0.5935986604093653, -0.5285277328950352, -0.6068764832493029,
> -1.1752312553140132, 0.19916734259906424, 0.976063912714949],
> [-0.5919650431077654, 0.5271408402306996, -0.6096719792636803,
> 1.2379670854947114, 0.2656040423387233, -0.972363043155988],
> [0.5914974900295467, 0.5266568950860249, -0.6105433925177587,
> 1.258297461476007, -0.285688537323182, 0.9726089241528251]]
> e = [0, 0, 0, 0, 0, -1]
> bounds = [(None, None), (None, None), (None, None), (None, None), (None,
> None), (0, 1)]
> b = [0]*len(A)
> result = linprog(e, A_ub = A, b_ub = b, bounds = bounds)
> print np.matmul(A, result.x)
>
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> --
> Chris Waigl . chris.waigl at gmail.com <mailto:chris.waigl at gmail.com> .
> chris at lascribe.net <mailto:chris at lascribe.net>
> http://eggcorns.lascribe.net . http://chryss.eu
>
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