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For questions about mathematical optimization problems involving both continuous and binary or general integer variables.
5
votes
Accepted
How to reformulate a discontinuous piecewise-quadratic functions
Although focused on implementing the model in YALMIP instead of CVX (converting the code should be trivial), precisely this case is described in the following tutorial https://yalmip.github.io/modelli …
6
votes
Index of element in MILP vector decision variable that equals 1
My interpretation is that you want $y$ to be $i$ if $p_i=1$. You can do that with a simple multiplication $y=c^Tp$ where the constant vector $c$ is given by $c_i=i$.
5
votes
MILP modelling on minimal disturbance of right-hand-side to make a linear system infeasible
By Farkas lemma, infeasibility of $Ax\leq b$ is equivalent to feasibility of $A^Ty = 0, y^Tb < 0, y\geq 0$, or more practically useful $A^Ty=0, y^Tb \leq -1, y\geq 0$.
Unfortunately, this will lead to …
6
votes
Accepted
Forbid transformation of max(x,y) into MILP
In YALMIP you can use arbitrary black-box operators to circumvent modelling
n = 5;
c = randn(3*n,1);
A = randn(10*n,3*n);
b = rand(10*n,1);
% MILP model
x = sdpvar(2*n,1);
Domain= [0 <= x <= 1]; % Bi …
7
votes
Modeling floor function exactly
As all have mentioned, the problem is intrinsically hard, as it effectively involves a strict inequality.
One way to represent it, using no strict inequalities or magic constants, is to use the simpl …
5
votes
Accepted
Why does one objective function prove feasibility faster than another?
Consider maximizing $\sum_{i=1}^n x_i$ subject to $x$ binary and $\sum_{i=1}^n i x_i \leq 2$
Solve relaxation and it gives $x_1 = 1, x_2 = 1/2$ with remaining variables 0. Branch on $x_2 = 0$ and the …
6
votes
Linearizing a quadratic function with more variables or not in Gurobi?
You can ask Gurobi to do this for you
https://www.gurobi.com/documentation/9.1/refman/preqlinearize.html
Whether a linearization leads to better or worse performance is almost impossible to know befor …
3
votes
Accepted
How to deal with log0 in optimization problem
You are not going to be able to add these logs and quadratic terms to the model via simple double-sided big-M constraints, as they generate non-convex use of convex quadratics and logs, and CVX does n …
3
votes
Accepted
How to transform this problem with logarithmic objective function into an approximated conve...
Introduce a term $y_m$ to replace and lower bound the terms inside the logarithm.
Those lower bound constraints simplify to
$\sum_{n=1}^{N}(1-x_{m,n})\omega_{m,n}+z\ge y_m(\sum_{n=1}^{N}x_{m,n}\omega_ …
5
votes
Accepted
Multiple If else constraints in Mixed integer programming
You just seem to have hidden a long list of constraints of the form $(x_i=j) \Rightarrow \text{equalities}_{ij}$
Introduce a binary matrix $C_{ij}$ with $\sum_j C_{ij}= 1$ and $C_{ij} \Rightarrow \{x …
5
votes
Accepted
Non-linear optimization local or global solution
Introduce a binary variable $\delta_t$ to represent which case it is and $z_t$ to represent the modelled product, and your MILP model of the piecewise-affine dynamics would be
${EP}_t\ =\ \sum_{i=1}^{ …