QUBO stands for quadratic unconstrained binary optimization, and is a problem type traditionally used in computer science. Variables are TRUE and FALSE, states that correspond to 1 and 0 values.
A QUBO problem is defined using an upper-diagonal matrix Q, which is an N x N upper-triangular matrix of real weights, and x, a vector of binary variables, as minimizing the function
where the diagonal terms, Q_i,i, are the linear coefficients and the nonzero off-diagonal terms are the quadratic coefficients Q_i,j.
This can be expressed more concisely as
In scalar notation, the objective function expressed as a QUBO is as follows: