# Tag Info

1 vote

### Ways to improve lower bounds while solving MIPs

There are serveral strategies. I recomend Lagrangian and Surrogate relaxation. Look for James Davis video "2.6 lagrangian relaxation". I guess it would help.
Accepted

### MIP formulation for a lower semi-continuous function

You want to maximize $\max(f(x),0)$. Assume $L \le f(x) \le U$ for constants $L$ and $U$. Maximize $g(x)$ subject to $$0 \le g(x) \le U y \\ 0 \le g(x) - f(x) \le (0-L)(1-y) \\ y \in \{0,1\}$$

### What's the best way to speed up Benders Decomposition for a stochastic vehicle routing problem?

Without knowing much about the problem, there are several approaches (listed in increasing difficulty of implementation): 1). Warm-start the model at a known feasible solution. 2). You could try ...

### How to design a constraint to control flow in a non-network optimization model

I actually solved this problem recently on my own, though my approach was not based entirely on operations research, but relied a bit on the logic of running the whole program. So if you are looking ...
1 vote
Accepted

### How to design a constraint to control flow in a non-network optimization model

Ok, suppose production process $i1$ goes to consumption process $i2$. You can create a map of $i1 -> i2$ & define parameters $z_{i1,i2}$such that if $i1 -> i2$, then $z_{i1,i2} = 1$, $0$ ...

### Will adding this constraint help my model?

I tried to test a simple example of what you proposed. I am setting an upper and lower bound $[0,5]$, $[0,4]$ to the variables $x$ and $z$ respectively. By turning off the pre-solving, heuristics, and ...
Accepted

### Will adding this constraint help my model?

I think the answer is a definitive "maybe". Adding constraint (3) might help the solver to recognize infeasible branches of the search tree sooner (branches with $\delta = 1$ where side ...

### Workforce scheduling problem

We typically handle "overlapping shifts", "minimum X hours between two shifts" and/or "one shift per day" as separate constraints. This is because in some cases they aren'...
Accepted

### Workforce scheduling problem

Consider a side-constrained network formulation in which you have a node for each employee-day-shift and a directed arc if the employees are the same, the days are consecutive, and the shifts do not ...
Accepted

### Linearize piecewise function without big-M constraints

I don't see or remember a way around the big-M constraints. Thankfully they are not that hard to write. Or you may use indicator constraints which are even simpler. We add new binary variables, and ...
1 vote

### Problems understanding model notation in LPs

The $\wedge$ simply means "and". So for example, constraint III is telling you to sum over all pair of $i$ and $t$ such that $i.NC = 1$ and $i.B=b$. As for what the authors mean by the "...
1 vote
Accepted

What you could do is the following: Introduce variable $t_{ij}$ that is the time between tasks $i$ and $j$. This variable is only defined for once per pair $ij$, therefore the $j >i$. Then, you can ...