Questions tagged [combinatorial-optimization]

For questions about optimization over a discrete solution space.

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1 vote
1 answer
38 views

Formulation to avoid partial coverage for demand points

I would like to assign items to a box over a time horizon where each box has an associated deadline. I was wondering if my formulation and / or variable definition can be improved. In other words, ...
3 votes
1 answer
259 views

Is evolution algorithm suitable for combinatorial optimization problem

I'm wondering if evolution-like (or genetic) algorithms are competitive for combinatorial optimization (CO) problems, such as knapsack, maximum independent set, travelling salesman etc. problems in ...
0 votes
1 answer
106 views

How to minimize number of machines required to serve tasks, and return the schedules for each machine?

The problem I'm trying to solve is the following: Given $t\in \{1,2,3....T\}$ tasks, integer. Tasks have release times $r_t$ and deadlines $d_t$, and processing times $p_t$, all continuous, real-...
2 votes
1 answer
105 views

How to change the variable bound from an interval to its lower and upper bound in JuMP?

When I read a model from a ".mps" file using "read_from_file" in JuMP and print it, I find that many bounds are written in the interval format like "x \in [0, 1]". I want ...
1 vote
1 answer
51 views

Is there any literature on constrained nonlinear optimization where constraint returns have to be queried from an oracle?

I am looking at literature considering constrained optimization problems of the form: $\min_{x\in X\subseteq R^n} f(x), \text{ subject to } g_{oracle}(x) \leq 0$ The optimization algorithm doesn't ...
1 vote
1 answer
100 views

Constraints to avoid disjointed solutions in a MIP

Given an directed graph $G= (N,E)$, where $N$ is the set of nodes and $E$ is the set of all edges, each associated with a direction. $G$ is a connected graph but not necessarily a complete graph. A ...
1 vote
2 answers
111 views

Persisting Subproblem Infeasibility Benders Decomposition

I'm currently working on an optimization problem where I'm using Benders decomposition to solve a complex problem involving the installation of charging stations. The master problem determines the ...
3 votes
1 answer
526 views

Packing a number of unequal circles in a rectangle

Given the dimension of a rectangle and the radii of n unequal circles, how can I decide if these circles can fit in the rectangle? I don't know if there is a formula to compute such a thing! The ...
2 votes
1 answer
81 views

Clustering optimization problem for categorical data: How to solve?

Problem Say I have a data set of $n \in \mathbb N$ survey responses to $N \in \mathbb N$ categorically-valued multiple-choice questions, which we denote as $$ \mathbf X = \{ \mathbf x_j \}_{j = 1}^n \...
0 votes
1 answer
48 views

Heuristics for selecting the right fleet combinations for a VRP

In most VRPs, we start from a set of fleets (homogeneous or heterogeneous) and a set of points to be served. Then there are other constraints that need to be taken care of. I am looking for a ...
0 votes
1 answer
78 views

Knapsack Problem with Multiple Properties

Problem Description I have many examples (say $N$) to choose from to train my machine learning model. However, I could only select $n \ll N$ based on a set of $K$ attributes $\{a_1, a_2, \cdots, a_k\}$...
1 vote
1 answer
92 views

Modelling Optimal Sorting Networks

I am trying to find sorting networks having the optimal depth or optimal number of comparators by generating $2^n$ binary sequences where $n$ is the channel size. The main variables in my model are as ...
1 vote
1 answer
82 views

Workforce scheduling MIP formulation

I am working on a large scale workforce scheduling problem with a large number of hard and soft constraints. The soft constraints are modeled as objectives with penalties for violating them. So my ...
3 votes
1 answer
178 views

Optimization challenges (currently active or soon to begin)

I am looking for an optimization challenge for an intern (such as ROADEF Challenge, Gecco competitions, or CEC competitions) active during summer 2023 (ideally July - September). Longer competitions ...
0 votes
0 answers
34 views

How to embed an arbitrary graph into (k,d)-kautz space (like multidimensional scaling of non-normed space)

How to embed an arbitrary graph into (k,d)-kautz space (like multidimensional scaling of non-normed space)? See details in the following. Given a graph $G = \{V,E\}$, we have a distance matrix (the ...
0 votes
0 answers
61 views

Loan Allocation Problem

I need help with the following use case: Context : Let's say there is a fintech "F", that helps with aggregating personal loans. Now, "F" can pool in customers who need a personal ...
3 votes
3 answers
248 views

Equivalence between constraints in ILP

Let's have binary variables $x$ and $y$. I'd like to define a helping binary variable $z$ such that $$ z = 1 \; \;\; \mathrm{iff} \; \; \; x + y = 2.$$ If I wanted to express the equivalence between ...
0 votes
1 answer
140 views

Converting a piecewise function to a linear equation as a constraint

The value of one of the variable of my model (alpha_1) is given by a piecewise function. Each element of the piecewise depends on the value of some other binary decision variables (X1, x2, x3). I'd ...
2 votes
2 answers
177 views

Parallel scheduling with precedence constraints and variable job length

I have $N$ jobs and $M$ machines and want to minimize the makespan, i.e. the total time to finish all jobs. Some jobs have precedence constraints and can only be started once other jobs are finished. ...
5 votes
6 answers
2k views

How can I solve this algorithmic task in Python?

I am trying to solve this task: There are three datasets: first data on offices in cities: each city has a certain number of offices and each office has its own capacity of employees. Second, data on ...
4 votes
0 answers
770 views

Google-OR tools vs Pyomo and other commercial Solvers for solving a simple maximum flow problem

I have implemented a Pyomo model for solving maximum flow problem as a subroutine of an algorithm. However, the approach does not scale very well because Pyomo does not provide a very good way to re-...
2 votes
3 answers
370 views

"Re-optimize" feature in Commercial Optimization

How does the "re-optimize" feature work in commercial optimization solvers such as IBM CPLEX and Gurobi? I recently experienced a considerable performance boost by re-solving a model with ...
2 votes
0 answers
48 views

A 'capacitated' VRP model where the capacity constraint is the time

I am trying to model a capacitated VRP model where the capacity is defined by the time the vehicle is available. What I have: List of vehicles available and the time for which they are available List ...
5 votes
1 answer
261 views

Optimize selection of metal sheets to keep in stock

I already asked this on stack overflow but just found this forum instead and figured it was more suited here. If this isn't allowed please feel free to tell me and I'll delete the post. I am doing ...
8 votes
3 answers
4k views

Can I use 'SCIP' solver for PYOMO?

I have an MINLP problem to solve where I was initially using 'ipopt' solver but the solution was not sticking to 'binary/boolean/integer' domain type for a variable. I am not sure which free solver ...
7 votes
1 answer
405 views

How to reformulate (linearize/convexify) a budgeted assignment problem?

I have a scheduling problem at hand. In my system, there is a service station with $M$ service outlets, therefore, the service station can serve $M$ users at a time. But, there are $N$ users $N>M$ ...
7 votes
1 answer
407 views

Can't solve a specific instance of Gilmore & Gomory Cutting Stock with Delayed Column Generation

I am now somewhat comfortable with delayed column generation, but one specific example has been bugging me, as it's quite simple (on the second iteration we stop) and I can't reach the optimal ...
4 votes
2 answers
558 views

Alternative way to restrict an employee to work on multiple jobs

Suppose I have a set of employee $E$ and set of jobs $J$ in a given time horizon $T$. I would like to make sure that no employee works on multiple jobs where each job $e\in E$ takes a certain amount ...
1 vote
1 answer
142 views

How to model this constraint? [duplicate]

I have a system with $C$ machine and $U$ users. The decision variables are $d_{u,c}$ and ${\bf f}_{u,c}$ with $c=1,2,\cdots,C$ and $u=1,2,\cdots,U$, where $$d_{u,c}\in\{0,1\}$$ $${\bf f}_{u,c}\in\...
6 votes
2 answers
194 views

Cover cuts for knapsack constraint with integer variables

It is known for knapsack type constraint $\sum_{i \in N} a_i x_i \leq b , x \in \{0,1\}$, we can generate the so called cover cuts that have the sum of coefficient in a set C greater than b. The cover ...
1 vote
2 answers
104 views

How do I approach this optimization problem?

I'm a software dev that's pretty new to OR and want to solve a specific problem I came across at my job. From what I've researched it is a variation of resource-constrained assignment problem, but ...
1 vote
1 answer
127 views

Further decomposing master problem in benders

In the benders decomposition, is it possible to decompose the master problem as follows and let the subproblem become the subproblem of the new subproblem? Let's say that we have the following problem....
3 votes
3 answers
378 views

Can I formulate a green vehicle routing problem with pyomo model then solve it with metaheuristics algorithms e.g. GA,HS rather than pyomo solver?

I want to use pyomo to formulate MILP problems in python such as Green Vehicle Routing Problem and others but use metaheuristics algorithms (GA, Harmony search, etc.) to solve those problems instead ...
2 votes
1 answer
204 views

Implementing subtour constraints to a VRP in python

Hi I have a problem related to the vehicle routing problem, where I want to implement the subtour constraints $\sum_{(i,j)\in S} x_{ij} \leq \lvert S \rvert - r(S), \: \forall S \subseteq N, S > 2$ ...
3 votes
1 answer
183 views

Assignment problem with mutually exclusive constraints has an integral polyhedron?

I have the following problem $\min \sum_{i\in I} \sum_{j \in J} c_{ij} x_{ij} $ $s.t. \sum_{j \in J} x_{ij} \leq b_i, \forall i \in I$ $\sum_{j \in S_l} x_{ij} \leq 1, \forall l \in L, i \in I $ $\...
3 votes
2 answers
281 views

Modelling a Capacitated Facility Location Problem in such a way that a few candidate locations are always selected

I am trying to model a MIP for a Capacitated Facility Location problem in a way that a number of selected candidate locations will always be there. How can I model it? My approach: Add a cost ...
-2 votes
1 answer
86 views

constraints for a zero-one integer programming problem

We want to arrange 8 tables in 2 rooms. How to write the following constraints? Either table 3 or table 6 must be in room 1 (or both). Exactly one of tables 7 and 8 must be in room 2.
6 votes
1 answer
349 views

Valid Inequality Example (Wolsey Example 9.3)

I was following Wolsey Example 9.3: Let $X = \{(x,y) \in (R^{m}_+,B^1) : \sum_{i=1}^m x_i \leq my\}$. Now consider the valid inequality $x_i \leq y$ and show that it is facet defining. My question is ...
1 vote
1 answer
49 views

Objective function for employee assignation problem solved with simulated annealing

Imagine you have a total number of employees assigned to a task. Each task requires N quantity of employees for it to be 100% efficient, something like this: Each employee is assigned to a task in ...
6 votes
1 answer
263 views

Column generation approach for CVRP

I want to use a column generation based heuristic to solve a capacitated Vehicle Routing Problem. I know the basics of the algorithm but I don't have much experience in coding. is there any code about ...
10 votes
3 answers
844 views

Modeling the Choose function

In statistics, one often encounters the choose function ${x \choose y}$ which encodes the number of ways of choosing $y$ items from a set of $x$ items. How would one go about modeling a choose ...
3 votes
0 answers
66 views

How can I use a local search meta heuristic after completing construction heuristic?

I'm investigating how I can combine a construction heuristic and a local search metaheuristic to quickly solve Capacitated Split Delivery Veichle Routing Problem (CSDVRP). The construction heuristic ...
2 votes
1 answer
103 views

Construction heuristic for SDCVRP

I'm not sure how I should formulate this question as precisely as possible but at the same time retaining as much specific information as possible but I'll have a go. I have implemented a (Split ...
6 votes
2 answers
201 views

When should we avoid linearizing a quadratic term?

Some solvers like Gurobi can handle mixed-integer quadratically-constrained quadratic models regardless of their nonconvexity. I have some experience that Gurobi can handle instances of the max $k$-...
19 votes
1 answer
465 views

Combinatorial Optimization: Metaheuristics, CP, IP -- "versus" or "and"?

"Recently" someone asked on Twitter whether "people still use genetic algorithms for integer programs". The "majority answer", i.e., 1 out of 1, was: "Yes" . So,...
5 votes
2 answers
908 views

Book to learn metaheuristics

I want to learn about metaheuristics and start building one to solve combinatorial optimization which is pretty hard to solve. I am looking for book recommendation or any learning experience to learn ...
3 votes
2 answers
420 views

How to formulate this in gurobi

I have subtour elimination constraint but can't understand and formulate this in gurobipy. How to formulate this ? $$ \sum_{i \in S} \sum_{j \in S} x_{i j}^{v} \leq|S|-1 \quad v=1, \ldots, m ; S \...
1 vote
2 answers
461 views

Subtour elimination in SDVRP

I have the model below, based on this paper. It's a vehicle routing problem with split deliveries, i.e the locations/customers can be visited by multiple vehicles that share the demand at that vehicle....
3 votes
0 answers
113 views

Hard time in understanding OR Research or Paper [closed]

I'm from soft-engineering background and really having a hard time when reading and understanding the formulation from a research or published paper. For example, when looking at a container loading ...
2 votes
0 answers
41 views

How best to compress a list of objective function evaluations in model-based optimization?

In Bayesian Optimization/other model-based numeric optimization methods, is there a useful way to compress or aggregate the observation history into a single value so you don't have to keep the result ...

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