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Xref: bloom-picayune.mit.edu news.answers:4542 sci.math.num-analysis:6341 Path: bloom-picayune.mit.edu!enterpoop.mit.edu!news.media.mit.edu!micro-heart-of-gold.mit.edu!rutgers!sun-barr!cs.utexas.edu!uunet!timbuk.cray.com!walter.cray.com!jwg From: jwg@cray.com (John W. Gregory) Newsgroups: news.answers,sci.math.num-analysis Subject: Linear Programming FAQ Summary: A List of Frequently Asked Questions about Linear Programming Keywords: FAQ, LP, Linear Programming Message-ID: <linear-programming-faq-1-724103080@cray.com> Date: 11 Dec 92 19:44:49 GMT Expires: 02/14/93 Reply-To: jwg@cray.com (John W. Gregory) Followup-To: sci.math.num-analysis Organization: Cray Research, Inc., Eagan MN USA Lines: 707 Approved: news-answers-request@MIT.Edu Originator: jwg@ceres Nntp-Posting-Host: ceres.cray.com Posted-By: auto-faq 2.4 Archive-name: linear-programming-faq Last-modified: 1992/12/11 Linear Programming - Frequently Asked Questions List (lp_faq) Most recent update: December 11, 1992 -------------------------------------------------------------------------- 0. "What's in this FAQ?" A: Table of Contents 0. "What's in this FAQ?" (Oh no! Is this a recursion?) 1. "What is Linear Programming?" 2. "Where is there a good code, preferably public domain, to solve Linear Programming problems?" 3. "Oh, and we also want to solve it as an integer program. I think there will be only a few thousand variables or so." 4. "I've written my own optimization code. Where are some test models?" 5. "What is MPS format?" 6. "What software is there for non-linear optimization?" 7. "What references are there in this field?" 8. "Just a quick question..." 9. "Who maintains this FAQ list?" -------------------------------------------------------------------------- 1. "What is Linear Programming?" A: A linear program (LP) is a problem that can be put into the form minimize cx subject to Ax=b x>=0 where x is a vector to be solved for, A is a matrix of known coefficients, and c and b are vectors of known coefficients. All these entities must have consistent dimensions, of course, and you can add "transpose" symbols to taste. The matrix A is generally not square, hence you don't solve an LP by just inverting A. Usually A has more columns than rows, so Ax=b is therefore underdetermined, leaving great latitude in the choice of x with which to minimize cx. Other formulations can be used in this framework. For instance, if you want to maximize instead of minimize, multiply the c vector by -1. If you have constraints that are inequalities rather than equations, you can introduce one new variable (a "slack") for each inequality and treat the augmented row of the matrix as an equation. LP codes will often take care of such "bookkeeping" for you. LP problems are usually solved by a technique known as the Simplex Method, developed in the 1940's and after. Briefly stated, this method works by taking a sequence of square submatrices of A and solving for x, in such a way that successive solutions always improve, until a point is reached where improvement is no longer possible. A family of LP algorithms known as Interior Point methods has been developed starting in the 1980's, that can be faster for many (but so far not all) problems. Such methods are characterized by constructing a sequence of trial solutions that go through the interior of the solution space, in contrast to the Simplex Method which stays on the boundary and examines only the corners (vertices). LP has a variety of uses, in such areas as petroleum, finance, transportation, forestry, and military. The word "Programming" is used here in the sense of "planning"; the necessary relationship to computer programming was incidental to the choice of name. Hence the phrase "LP program" to refer to a piece of software is not a redundancy, although I tend to use the term "code" instead to avoid the possible ambiguity. -------------------------------------------------------------------------- 2. "Where is there a good code, preferably public domain, to solve Linear Programming problems?" A: It depends on the difficulty of your models. LP technology and computer technology have both made such great leaps that models that were previously considered "large" are now routinely solved. Nowadays, with good commercial software, models with a few thousand constraints and several thousand variables can be tackled with a PC. Workstations can often handle models with variables in the tens of thousands, or even greater. It's hard to be specific about sizes and speed, a priori, due to the wide variation in things like model structure and variation in factorizing the basis matrices. There is a recently released public domain code, written in C, called "lp_solve" that is available on Usenet in the "comp.sources.reviewed" newsgroup. Its author (Michel Berkelaar, email at michel@es.ele.tue.nl) claims to have solved models with up to 30,000 variables and 50,000 constraints. My own experience with this code is not quite so uniformly optimistic (new users of LP are sometimes shocked to learn that just because a given code has solved a model of a given dimension, it may not be able to solve all models of the same size). Still, for someone who isn't sure just what kind of LP code is needed, it represents a very reasonable first try, and the price is certainly right. The code is archived at anonymous ftp site "ftp.uu.net", in directory "/usenet/comp.sources.reviewed/volume02/lp_solve". It consists of three files, part00.Z, part01.Z and part02.Z. You should download them in binary mode, and use the `uncompress` utility to expand them to normal ASCII format. The file called part00 contains reviewers' comments, and the other two files can be unpacked by removing the first 9 lines and executing the files as shell scripts (e.g., `sh part01`). Then follow the instructions in the README and INSTALL files. For DOS/PC users, Prof. Timo Salmi at the University of Vaasa in Finland offers a code called "tslin". You should be able to access it by ftp at garbo.uwasa.fi in directory /pc/ts (the current file name is tslin33b.zip, apparently using ZIP compression), or else I suggest contacting Prof. Salmi at ts@uwasa.fi. The consensus is that the LP code published in Numerical Recipes is not at all strong, and should be avoided for heavy-duty use. If your requirement is for a solver that can handle 100-variable models, it might be okay. There is an ACM TOMS routine for LP, #552, available from the netlib server, in directory /netlib/toms. See the section on test models for detail on how to use this server. If you have access to one of the commercial math libraries, such as IMSL or NAG, you may be able to use an LP routine from there. If your models prove to be too difficult for free software to handle, then you can consider acquiring a commercial LP code. There are dozens of such codes on the market. I have my own opinions, but for reasons of space, generality and fairness, I will not attempt even to list the codes I know of here. Instead I refer you to the annual survey of LP software published in "OR/MS Today", a joint publication of ORSA (Operations Research Society of America) and TIMS (The Institute of Management Science). I think it's likely that you can find a copy of the June, 1992 issue, either through a library, or by contacting a member of these two organizations (most universities probably have several members among the faculty and student body). The survey lists almost fifty actively marketed products. This publication also carries advertisements for many of these products, which may give you additional information to help make a decision. There are many considerations in selecting an LP code. Speed is important, but LP is complex enough that different codes go faster on different models; you won't find a "Consumer Reports" article 8v) to say with certainty which code is THE fastest. I usually suggest getting benchmark results for your particular type of model if speed is paramount to you. Benchmarking may also help determine whether a given code has sufficient numerical stability for your kind of models. Other questions you should answer: Can you use a stand-alone code, or do you need a code that can be used as a callable library, or do you require source code? Do you want the flexibility of a code that runs on many platforms and/or operating systems, or do you want code that's tuned to your particular hardware architecture (in which case your hardware vendor may have suggestions)? Is the choice of algorithm (Simplex, Interior Point) important to you? Do you need an interface to a spreadsheet code? Is the purchase price an overriding concern? Is the software offered at an academic discount (assuming you are at a university)? How much hotline support do you think you'll need? It may not always be true that "you get what you pay for," but it is rare that you get more than you pay for. 8v) There is usually a large difference in LP codes, in performance (speed, numerical stability, adaptability to computer architectures) and features, as you climb the price scale. If a code seems overpriced to you, you may not yet understand all of its features. -------------------------------------------------------------------------- 3. "Oh, and we also want to solve it as an integer program. I think there will be only a few thousand variables or so." A: Hmmmm. You want - Nontrivial model size - Integer solutions - Public domain code Pick one or maybe two of the above. You can't have all three. 8v) Integer LP models are ones where the answers must not take fractional values. It may not be obvious that this is a VERY much harder problem than ordinary LP, but it is nonetheless true. The buzzword is "NP- Completeness", the definition of which is beyond the scope of this document but means in essence that in the worst case the amount of time to solve a family of related problems goes up exponentially as the size of the problem grows. Integer models may be ones where only some of the variables are to be integer and others may be real-valued (termed "Mixed Integer LP" or MILP, or "Mixed Integer Programming" or MIP), or they may be ones where all the variables must be integral (termed "Integer LP" or ILP). The class of ILP is often further subdivided into problems where the only legal values are {0,1} ("Binary" or "Zero-One" ILP), and general integer problems. For the sake of generality, the Integer LP problem will be referred to here as MIP, since the other classes can be viewed as special cases of MIP. You should be prepared to solve far smaller MIP models than the corresponding LP model, given a certain amount of time you wish to allow (unless you and your model happen to be very lucky). There exist models that are considered challenging, with mere hundreds of variables. Conversely, some models with tens of thousands of variables solve readily. It all depends, and the best explanations of "why" always seem to happen after the fact. 8v) One exception to this gloomy outlook is that there are certain models whose LP solution always turns out to be integer. Best known of these are the so-called Transportation Problem, Assignment Problem, and Network-Flow Problem. It turns out that these problems are best solved by specialized routines that take major shortcuts in the Simplex Method, and as a result are relatively quick running. See the section on references for a book by Kennington and Helgason, which contains some source code for Netflo. Netflo is available by anonymous ftp at dimacs.rutgers.edu, in directory /pub/netflow/mincost/solver-1, but I don't know the copyright situation (I used to think you had to buy the book to get the code). People are sometimes surprised to learn that MIP problems are solved using floating point arithmetic. Although various algorithms for MIP have been studied, most if not all available general purpose large-scale LP codes use a method called "Branch and Bound" to try to find an optimal solution. In a nutshell, B&B solves MIP by solving a sequence of related LP models. Good codes for MIP distinguish themselves more by solving shorter sequences of LP's, than by solving the individual LP's faster. Even moreso than with regular LP, a costly commercial code may prove its value to you if your MIP model is difficult. As a point of interest, the Simplex Method currently retains an advantage over the newer Interior Point methods for solving these sequences of LP's. The public domain code "lp_solve", mentioned earlier, accepts MIP models, as do a large proportion of the commercial LP codes in the OR/MS Today survey. I have seen mention made of algorithm 333 in the Collected Algorithms from CACM, though I'd be surprised if it was robust enough to solve large models. The better MIP codes have numerous parameters and options to give the user control over the solution strategy. Most have the capability of stopping before an optimum is proved, printing the best answer obtained so far. For many MIP models, stopping early is a practical necessity. Fortunately, a solution that has been proved by the algorithm to be within, say, 1% of optimality often turns out to be the true optimum, and the bulk of the computation time is spent proving the optimality. For many modeling situations, a near-optimal solution is acceptable anyway. Once one accepts that large MIP models are not typically solved to a proved optimal solution, that opens up a broad area of approximate methods, probabilistic methods and heuristics, as well as modifications to B&B. Claims have been made for Genetic Algorithms and Simulated Annealing, though (IMHO) these successes have been problem dependent and difficult to generalize. (A reference for GA is David Goldberg, "Genetic Algorithms in Machine Learning.") Whatever the solution method you choose, when trying to solve a difficult MIP model, it is usually crucial to understand the workings of the physical system (or whatever) you are modeling, and try to find some insight that will assist your chosen algorithm to work better. A related observation is that the way you formulate your model can be as important as the actual choice of solver. You should consider getting some assistance if this is your first time trying to solve a large (>100 variable) problem. -------------------------------------------------------------------------- 4. "I've written my own optimization code. Where are some test models?" A: In light of the comments above, I hope your aims are fairly modest, for there are already a lot of good codes out there. I hope your LP code makes use of sparse matrix techniques, rather than using a tableau form of the Simplex method, because the latter usually ends up being numerically unstable and very slow. If you want to try out your code on some real-world LP models, there is a very nice collection of small-to-medium-size ones on netlib. If you have ftp access, you can try "ftp research.att.com", using "netlib" as the Name, and your email address as the password. Do a "cd lp/data" and look around. There should be a "readme" file, which you would want to look at first. Alternatively, you can reach an e-mail server via "netlib@ornl.gov", to which you can send a message saying "send index from lp/data"; follow the instructions you receive. The Netlib LP files (after you uncompress them) are in a format called MPS, which is described in another section of this document. There is a collection of MIP models, housed at Rice University. Send an email message containing "send catalog" to softlib@rice.edu, to get started. There is a collection of network-flow codes and models at DIMACS (Rutgers University). Use anonymous FTP at dimacs.rutgers.edu. Start looking in /pub/netflow. Another network generator is called NETGEN and is available on netlib (lp/generators). John Beasley has posted information on his OR-Lib, which contains various optimization test problems. Send e-mail to umtsk99@vaxa.cc.imperial.ac.uk to get started. Or have a look in the Journal of the Operational Research Society, Volume 41, Number 11, Pages 1069-72. There are various test sets for NLP (Non-Linear Programming). Among those I've seen mentioned are - ACM TOMS (Transactions on Mathematical Software), V13 No3 P272 - publications (listed in another section of this list) by Schittkowski; Hock & Schittkowski; Floudas & Pardalos; Torn; Hughes & Grawiog. Many of the other references also contain various problems that you could use to test a code. The modeling language GAMS comes with about 100 test models, which you might be able to test your code with. -------------------------------------------------------------------------- 5. "What is MPS format?" A: MPS format was named after an early IBM LP product and has emerged as a de facto standard ASCII medium among most of the various commercial LP codes. You will need to write your own reader routine for this, but it's not too hard. The main things to know about MPS format are that it is column oriented (as opposed to entering the model as equations), and everything (variables, rows, etc.) gets a name. MPS format is described in more detail in Murtagh's book, referenced in another section. Here is a little sample model, explained in more detail below: NAME METALS ROWS N VALUE E YIELD L FE L MN L CU L MG G AL L SI COLUMNS BIN1 VALUE 0.03 YIELD 1. BIN1 FE 0.15 CU .03 BIN1 MN 0.02 MG .02 BIN1 AL 0.7 SI .02 BIN2 VALUE 0.08 YIELD 1. BIN2 FE .04 CU .05 BIN2 MN .04 MG .03 BIN2 AL .75 SI .06 BIN3 VALUE 0.17 YIELD 1. BIN3 FE .02 CU .08 BIN3 MN .01 AL .8 BIN3 SI .08 BIN4 VALUE 0.12 YIELD 1. BIN4 FE .04 CU .02 BIN4 MN .02 AL .75 BIN4 SI 0.12 BIN5 VALUE 0.15 YIELD 1. BIN5 FE .02 CU .06 BIN5 MN .02 MG .01 BIN5 SI .02 AL .8 ALUM VALUE 0.21 YIELD 1. ALUM FE .01 CU .01 ALUM AL .97 SI .01 SILCON VALUE 0.38 YIELD 1. SILCON FE .03 SI .97 RHS ALOY1 YIELD 2000. FE 60. ALOY1 CU 100. MN 40. ALOY1 MG 30. AL 1500. ALOY1 SI 300. BOUNDS UP PROD1 BIN1 200.00 UP PROD1 BIN2 750.00 LO PROD1 BIN3 400.00 UP PROD1 BIN3 800.00 LO PROD1 BIN4 100.00 UP PROD1 BIN4 700.00 UP PROD1 BIN5 1500.00 ENDATA MPS is an old format, so it is set up as though you were using punch cards, and is not free format. Fields start in column 1, 5, 15, 25, 40 and 50. Sections of an MPS file are marked by so-called header cards, which are distinguished by their starting in column 1. Although it is typical to use upper-case throughout the file (like I said, MPS has long historical roots), many MPS-readers will accept mixed-case for anything except the header cards, and some allow mixed-case anywhere. The NAME card can be anything you want. The ROWS section defines the names of all the constraints; entries in column 2 or 3 are E for equality rows, L for less-than ( <= ) rows, G for greater-than ( >= ) rows, and N for non- constraining rows (the first of which would be interpreted as the objective function). The largest part of the file is in the COLUMNS section, which is the place where the entries of the A-matrix are put. All entries for a given column must be placed consecutively, although within a column the order of the entries (rows) is irrelevant. Rows not mentioned for a column are assumed to have a coefficient of zero. The RHS section allows one or more right-hand-side vectors to be defined; most people don't bother having more than one. In the above example, its name is ALOY1, and has non-zero values in all 7 of the constraint rows of the problem. Rows not mentioned are assumed to have a right-hand-side of zero. The optional BOUNDS section lets you put lower and upper bounds on individual variables (no * wildcards, unfortunately), instead of having to define extra rows in the matrix. All the bounds that have a given name in column 5 are taken together as a set. Variables not mentioned in a BOUNDS set are taken to be non-negative. There is another optional section called RANGES that I won't go into here. The final card must be the ENDATA, and yes, it is spelled funny. For comparison, here is the same model written out in equation format. Minimize VALUE: 0.03 BIN1 + 0.08 BIN2 + 0.17 BIN3 + 0.12 BIN4 + 0.15 BIN5 + 0.21 ALUM + 0.38 SILCON Subject To YIELD: BIN1 + BIN2 + BIN3 + BIN4 + BIN5 + ALUM + SILCON = 2000 FE: 0.15 BIN1 + 0.04 BIN2 + 0.02 BIN3 + 0.04 BIN4 + 0.02 BIN5 + 0.01 ALUM + 0.03 SILCON <= 60 MN: 0.02 BIN1 + 0.04 BIN2 + 0.01 BIN3 + 0.02 BIN4 + 0.02 BIN5 <= 40 CU: 0.03 BIN1 + 0.05 BIN2 + 0.08 BIN3 + 0.02 BIN4 + 0.06 BIN5 + 0.01 ALUM <= 100 MG: 0.02 BIN1 + 0.03 BIN2 + 0.01 BIN5 <= 30 AL: 0.7 BIN1 + 0.75 BIN2 + 0.8 BIN3 + 0.75 BIN4 + 0.8 BIN5 + 0.97 ALUM >= 1500 SI: 0.02 BIN1 + 0.06 BIN2 + 0.08 BIN3 + 0.12 BIN4 + 0.02 BIN5 + 0.01 ALUM + 0.97 SILCON <= 300 and 0 <= BIN1 <= 200 0 <= BIN2 <= 750 400 <= BIN3 <= 800 100 <= BIN4 <= 700 0 <= BIN5 <= 1500 -------------------------------------------------------------------------- 6. "What software is there for non-linear optimization?" A: I don't claim as much expertise in this area, but the question is frequent enough to be worth addressing. If I get enough feedback, this section might grow large enough to need to be split off as a separate FAQ list. It's unrealistic to expect to find one general NLP code that's going to work for every kind of nonlinear model. Instead, you should try to find a code that fits the problem you are solving. Nonlinear solution techniques can be divided into various categories, such as unconstrained, linearly constrained, convexly constrained, or general. If your problem doesn't fit in any category except "general", or you insist on a proven optimal solution (except when there no chance of multiple local minima), you should be prepared to have to use a method that boils down to exhaustive search, i.e., you have an intractable problem. See the comments in the MIP section on Simulated Annealing and Genetic Algorithms. Several of the commercial LP codes referenced above have specialized routines, particularly for Quadratic Programming (QP, linear constraints with a quadratic objective function). Many nonlinear problems can be solved (or at least confronted) by application of a sequence of LP or QP approximations. There is an ACM TOMS routine for QP, #587, available from the netlib server, in directory /netlib/toms. See the section on test models for detail on how to use this server. There is a directory, /netlib/opt, on the netlib server containing a collection of optimization routines. At my last check, I saw - "praxis" (unconstrained optimization, without requiring derivatives) - "tn" (Newton method for unconstrained or simple-bound optimization) - "ve08" (optimization of unconstrained separable function). - "simann" (unconstrained optimization using Simulated Annealing) - "vfsr" (constrained optimization using Simulated Annealing) Again, see the section on test models for detail on how to use this server. Here is a summary of codes mentioned in newsgroups in the past year, not sorted into categories. - MINOS - Stanford University, Office of Technology Licensing, 415-723-0651. This code is often used by researchers as a "benchmark" for others to compare with. - NPSOL - Stanford University, Office of Technology Licensing, 415-723-0651. - QPSOL - Stanford University, Office of Technology Licensing, 415-723-0651. - NAG Library routine E04UCF. - IMSL routine UMINF and UMIDH. - Harwell Library routine VF04. - Hooke and Jeeves algorithm - reference? - MINPAK I and II - Contact Steve Wright, wright@mcs.anl.gov - GENOCOP - Genetic algorithm, Zbigniew Michalewicz, zbyszek@mosaic.uncc.edu said to be available by ftp at unccsun.uncc.edu (152.15.10.88). - DFPMIN - Numerical Recipes (Davidon-Fletcher-Powell) - Amoeba - Numerical Recipes - Brent's Method - Numerical Recipes - FSQP - Contact Andre Tits, andre@src.umd.edu. Said to be free of charge to academic users. - CONMIN - Vanderplaats and Associates, Goleta CA - NOVA - DOT Products, Houston TX - GRG2 - Leon Lasdon, University of Texas, Austin TX - GINO - LINDO Systems, Chicago, IL -------------------------------------------------------------------------- 7. "What references are there in this field?" A: Too many to count. Here are a few that I like or have been recommended on the net. I have *not* reviewed them all. General reference [1] - Nemhauser, Rinnooy Kan, & Todd, eds, Optimization, North-Holland, 1989. (Very broad-reaching, with large bibliography. Good reference; it's the place I tend to look first. Tough sledding for beginners.) LP - Chvatal, Linear Programming, Freeman, 1983. (I find it hard to whole- heartedly recommend any one LP textbook, but this is one I'd probably use in teaching an undergraduate course.) - Hughes & Grawiog, Linear Programming: An Emphasis on Decision Making, Addison-Wesley, 1973. - Luenberger, Introduction to Linear and Nonlinear Programming, Addison Wesley, 1984. (Updated version of an old standby.) - Murtagh, B., Advanced Linear Programming, McGraw-Hill, 1981. (Good one after you've read an introductory text.) - Schrijver, Theory of Linear and Integer Programming, Wiley. - Williams, H.P., Model Building in Mathematical Programming, Wiley 1985. (Little on algorithms, but excellent for learning what makes a good model.) Interior Point LP - Marsten, et al., "Interior point methods for linear programming", Interfaces, pp 105-116, July-August 1990. (Introductory article, written by authors of a good commercial code.) - Marsten, et al., article to appear in ORSA Journal on Computing, 1993. (The latest results; a tech report version may be available sooner.) - Wright, M., "Interior methods for constrained optimization", Acta Mathematica, Cambridge University Press, 1992. (Survey article.) There is also a bibliography (with over 1000 entries!?!) obtainable by mailing to "netlib@ornl.gov" and saying "send intbib.bib from bib". Nonlinear Programming (can someone help classify these more usefully?) - Bazaraa & Shetty, Nonlinear Programming, Theory & Applications. - Coleman & Li, Large Scale Numerical Optimization, SIAM Books. - Dennis & Schnabel, Numerical Methods for Unconstrained Optimization and Nonlinear Equations, Prentice Hall, 1983. - Fiacco & McCormick, Sequential Unconstrained Minimization Techniques, SIAM Books. (An old standby, given new life by the interior point LP methods.) - Fletcher, R., Practical Methods of Optimization, Wiley 1987. (Good reference for Quadratic Programming, among other things.) - Floudas & Pardalos, A collection of test problems for constrained optimization algorithms, Springer-Verlag, 1990. - Gill, Murray & Wright, Practical Optimization, Academic Press, 1981. (An instant NLP classic when it was published.) - Hock & Schittkowski, Test Examples for Nonlinear Programming Codes, Springer-Verlag, 1981. - Kahaner, Moler & Nash, Numerical Methods and Software, Prentice-Hall. - More, "Numerical Solution of Bound Constrained Problems", in Computational Techniques & Applications, CTAC-87, Noye & Fletcher, eds, North-Holland, 29-37, 1988. - More & Toraldo, Algorithms for Bound Constrained Quadratic Programming Problems, Numerische Mathematik 55, 377-400, 1989. - Schittkowski, Nonlinear Programming Codes, Springer-Verlag, 1980. - Torn & Zilinskas, Global Optimization, Springer-Verlag, 1989. Other publications - Forsythe, Malcolm & Moler, Computer Methods for Mathematical Computations, Prentice-Hall. -Hansen, Global Optimization Using Interval Analysis, Marcel Dekker, 1991(?). (I'd be interested if anyone has any opinions on this one.) - Kennington & Helgason, Algorithms for Network Programming, Wiley, 1980. (A special case of LP.) - Kirkpatrick, Gelatt & Vecchi, Optimization by Simulated Annealing, Science, 220 (4598) 671-680, 1983. - Press, Flannery, Teukolsky & Vetterling , Numerical Recipes, Cambridge, 1986. (Comment: use with care.) -------------------------------------------------------------------------- 8. "Just a quick question..." Q: What is a matrix generator? A: This is a code that creates input for an LP (or MIP, or NLP) code, using a more natural input than MPS format. There are no free ones. Big names in this area are GAMS (Scientific Press), LINGO (LINDO Systems), and AMPL (information is in netlib/opt on the Netlib server). These products have links to various solvers (commercial and otherwise). Q: How do I diagnose an infeasible LP model? A: A model is infeasible if the constraints are inconsistent, i.e., if no feasible solution can be constructed. It's often difficult to track this down. The cure may even be ambiguous: is it that some demand was set too high, or a supply set too low? A useful technique is Goal Programming, one variant of which is to include two explicit slack variables (positive and negative) with huge cost coefficients, in each constraint. The revised model is guaranteed to have a solution, and you can look at which rows have slacks that are included in the "optimal" solution. By the way, I recommend a Goal Programming philosophy even if you aren't having trouble with feasibility; "come on, you could probably violate this constraint for a price." Q: I want to know specifically which constraints contradict each other. A: This may not be a well posed problem. If by this you mean you want to find the minimal set of constraints that should be removed to restore feasibility, this can be modeled as an Integer LP (which means, it's potentially a hard problem). Start with a Goal Programming approach as outlined above, and introduce some 0-1 variables to turn the slacks off or on. Then minimize on the sum of these 0-1 variables. Q: I just want to know whether or not there *exists* a feasible solution. A: Finding out if a model has a feasible solution is essentially as hard as finding the optimal solution (within a factor of 2 on average, in terms of effort in the Simplex Method). There are no shortcuts in general, unless you know something useful about your model's structure. Q: I have an LP, except it's got several objective functions. Help! A: This is indeed a difficult class of model. Fundamental to it is that there may be no unique solution. Approaches that have worked are Goal Programming (treat the objectives as constraints with costed slacks), Pareto preference analysis, and forming a composite objective from the real ones. There is a section on this whole topic in Reference [1]. My general advice is to attempt to cast your model in terms of dollars and cents wherever possible; sometimes the multiple objectives disappear! 8v) Q: I have an LP that has large almost-independent matrix blocks that are linked by a few constraints. Can I take advantage of this? A: In theory, yes. See section 6.2 in Reference [1] for a discussion of Dantzig-Wolfe decomposition. However, I am unaware of any commercial codes that will help you do this, so you'll have to create your own framework and then call your chosen LP solver to solve the subproblems. The folklore is that generally such schemes take such a long time to converge that they're slower than just solving the model as a whole. My advice, unless your model is so huge that a good solver can't handle it, is to not bother decomposing it. (It's probably more cost effective to upgrade your solver, if that's why you can't solve it, than to invest your time.) Q: I need to find all integer points in a polytope. A: There is no known way of doing this efficiently. A related question is how to find all the vertices of an LP, with the same pessimistic answer. The book by Schrijver is said to discuss this. Q: Are there any parallel LP codes? A: IBM has announced a parallel Branch and Bound capability in their package OSL, for use on clusters of workstations. "This is real, live commercial software, not a freebie. Contact forrest@watson.ibm.com". Jeffrey Horn (horn@cs.wisc.edu) recently compiled a bibliography of papers relating to research on parallel B&B. I'm not aware of any implementations (beyond the "toy" level) of simplex or interior-point solvers on parallel machines. Q: From a roll of cloth of given width and unlimited length, cut out ... A: You are referring to the Cutting Stock (or Trim Loss) problem. It is in most LP textbooks, or Reference [1]. I think it's an interesting problem, because the model formulation hinges on how you generate the variables of the eventual LP; you can't really just write down the equations. This trick of so-called Column Generation is used in diverse other problems such as airline crew scheduling. A related idea, Constraint Generation, is used to solve the Traveling Salesman Problem. Q: I am trying to solve a Traveling Salesman Problem ... A: I knew you'd ask that. Look at the bibliography in the Integer Programming section of Reference [1], particularly the ones with the names Groetschel and/or Padberg in them. TSP is a famously hard problem that has attracted many of the best minds in the field. I don't believe there are any commercial products to solve this problem. Q: I heard about this new Russian algorithm for Traveling Salesman Problems. A: You are speaking of Khachian's method for LP, published in 1979. It was the first LP algorithm to have a polynomial bound on the amount of work. Though important theoretically, it has had no impact in practice, because the polynomial bounds are huge. It works by surrounding the solution space with a sequence of shrinking ellipsoids. There continues to be research done on this method, for NLP. The connection to TSP is false, brought about by an erroneous New York Times article back then. Q: I need to do post-optimal analysis. A: Many commercial LP codes have features to do this. Also called Ranging or Sensitivity Analysis, it gives you information about how much the coefficients in the problem could change without affecting the nature of the solution. Most LP textbooks, such as Reference [1], describe this. -------------------------------------------------------------------------- 9. "Who maintains this FAQ list?" A: John W. Gregory LP Specialist (it says that on my business card, it must be true!) Applications Department Cray Research, Inc. Eagan, MN 55121 USA jwg@cray.com 612-683-3673 I suppose I should say something here to the effect that "the material in this document does not reflect any official position taken by Cray Research, Inc." Also, "use at your own risk", "no endorsement of products mentioned", etc., etc. I probably should have scattered more "IMHO"s around in the text, but that acronym seems weaselly and once you start it's hard to know where to stop. I should have put in a few more smilies here and there too, to assist the humor impaired - be on your toes. 8v) I've tried to keep my own biases (primarily, toward the high end of computing) from dominating what I write here, and other viewpoints that I've missed are welcome. Suggestions, corrections, topics you'd like to see covered, and additional material (particularly on NLP) are solicited. Comments to the effect "who died and made *you* optimal?" will likely result in you getting stuck with maintaining the FAQL instead of me. 8v) I regret that when I started this list I didn't keep careful track of all the contributors whose knowledge I tapped. In several instances, the information herein comes from postings on the net, which I assumed to be fair use and in the public domain. It being too late now to make individual acknowledgements, I offer a blanket THANKS to all who have posted on these topics to the net. This FAQL is also being posted to news.answers, which is archived in the periodic posting archive on pit-manager.mit.edu [18.172.1.27], in the anonymous ftp directory /pub/usenet/news.answers. Copies of this FAQL may be made freely, as long as it is distributed at no charge and with this disclaimer included. -------------------------------------------------------------------------- END lp_faq