# Operations Research Stack Exchange Community Digest

## Top new questions this week:

### Restoring a list from differences

Given a list of (absolute valued) pair differences ordered and with duplicates removed, how can we recover/reconstruct the list that generated these differences? We do not know anything about the ...

combinatorial-optimization difference-lists picat

### How fair is limit solver threads when publishing?

Reading gurobi reference manual I found this: We have also found that certain classes of MIP models benefit from reducing the thread count, often all the way down to one thread. Starting multiple ...

### Column generation for TSP

For teaching purposes, I would like to solve the Travelling Salesman Problem with a column generation approach. In the academic literature, an approach is proposed (for example here), where columns ...

modeling column-generation traveling-salesman dantzig-wolfe-decomposition

### Convex Optimization: Separation of Cones

I am trying to solve Exercise 2.39 at Boyd and Vandenberghe's Convex Optimization book. In one source, the answer is given as: 2.39 Separation of cones. Let $K$ and $\tilde K$ be two convex cones ...

convexity convex-optimization cone

### Operations Research books by world-famous authors made available on the web, despite not being "completed"

Are there any books on Operations Research subjects by world-famous authors which have been made available for free access on the web, despite never having been "completed" to the standard to which ...

online-resources books manuscripts

### Why is there a constant in the objective function of the *best subset selection problem*?

This article presents the following formulation of the best subset selection problem $$\min_{\|\beta\|_0\leq k}\frac{1}{2}\|y-X\beta\|^2_2$$ I wonder where the $1/2$ comes from. Help appreciated.

### Precedence Constraint in OR-Tools

I'm trying to write this constraint in C++ and add it to my model (addLessOrEqual): $$\text{start}_{i}+\text{duration}_{i}\le\text{start}_{j}$$ for an $(i,j)$ arc of precedence. Starts (IntVar) and ...

constraint-programming or-tools c++

## Greatest hits from previous weeks:

### How to determine the correct level of detail when modelling?

"All models are wrong, but some are useful" - George E. P. Box I usually work on what one could call operational problems. There I usually do not have too much trouble figuring out the level of ...

modeling strategic-planning

### "Best practices" for formulating MIPs

Often there are many alternatives ways for formulating a MIP. For example: The model contains inequality constraints that must hold with equality in an optimal solution. The model contains ...

mixed-integer-programming linear-programming solver cplex

### What is a "hard problem" in the context of Mixed-integer programming?

As a practical (real-world problems) point of view, it's important we could solve optimization problems as quickly as possible (for instance, to release a daily schedule). Maybe a problem with many ...

mixed-integer-programming optimization modeling

### Soft constraints and hard constraints

The terms "soft constraints" and "hard constraints" are used in the context of optimization modeling. Is there any standard way to figure out which is which in some of the complicated models?

optimization modeling constraint

### What are common and not so common abbreviations in Operations Research?

Many abbreviations are used in Operations Research. For example, we have abbreviations for problem classes (LP, MIP), solution methods (IPM), and specific problems (TSP, VRP). Which abbreviations are ...

terminology

### Is there a SQL/English like language that lets you define formulations given some data?

It would be very useful for beginning and non technical users to be able to define models in a way that was natural for them. Further this could perhaps assist generating some kind of generic ...

mixed-integer-programming applications modeling

### Optimization terminology: "Exact" v. "Approximate"

In optimization literature, I frequently see solution methods termed "exact" or "approximate". (I use the term "method" here because I suspect exactness, or its lack, is a function of both algorithm ...

combinatorial-optimization terminology

## Can you answer these questions?

### Gurobi stuck at start - 16GB RAM exhausted

I'm solving a large instance using Gurobi 7.0, 64-bit version on machine with 16GB of RAM. Gurobi is stuck at the start and all the memory is exhausted. Here is the output: 5378534 variables, all ...

integer-programming gurobi

### Demonstrating a given solution is basic for a MCFP?

Given a minimum-cost flow problem, how could I go about demonstrating that a specific solution (as sets of basic and non-basic (!) arcs that build a rooted spanning tree for a given graph) is basic ...

simplex network-flow