bugNESSY - Bugs: bug #22658, The parameter constraint algorithm...

 
 
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bug #22658: The parameter constraint algorithm is non-functional and can kill optimisation.

Submitted by:  Edward d Auvergne <bugman>
Submitted on:  Thu Sep 18 14:33:45 2014  
 
Category: NESSY source codeSeverity: 4 - Important
Priority: 7 - HighStatus: Postponed
Privacy: PublicAssigned to: None
Open/Closed: Open

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Thu Sep 18 14:33:45 2014, original submission:

This problem was discussed here:

https://mail.gna.org/public/nessy-users/2014-03/msg00004.html

The issue is that constraints are implemented as follows:

# kex
if kex < Constraints_container.kex[1][0] or kex > Constraints_container.kex[1][1]:
for j in range(0, len(err)):
err[j] = (err[j])*2

Here err is the chi2 value. The chi2 value is squared when a constraint is violated. This is not a known constraint algorithm and it actually introduces a jump discontinuity (https://en.wikipedia.org/wiki/Classification_of_discontinuities). Say that at the parameter limit is a boundary where chi2 = 2. If you are just inside the boundary, the value is 2. If you are just outside the value is now 4.

Discontinuities are fatal for almost all optimisation algorithms, and certainly for all algorithms used by NESSY.

The solution would be to implement a proper constraint algorithm. The simplest would be the log-barrier algorithm (https://en.wikipedia.org/wiki/Barrier_function#Logarithmic_barrier_function). A more powerful algorithm would be the Method of Multipliers, also known as the Augmented Lagrangian (https://en.wikipedia.org/wiki/Augmented_Lagrangian_method). These are both available via the minfx project (https://gna.org/projects/minfx/).

Edward d Auvergne <bugman>
Project Administrator

 

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