Max-Cut Problem

Given an undirected graph $G=(V,E)$, the Max-Cut problem aims to partition the node set $V$ into two disjoint subsets $S$ and $\overline{S}$ so that the number of edges in $E$ that have one endpoint in $S$ and the other in $\overline{S}$ is maximized.

Assume that the nodes are labeled $0,1,\ldots,n-1$. We introduce $n$ binary variables $x_0, x_1, \ldots, x_{n-1}$, where $x_i=1$ if and only if node $i$ belongs to $S$ ($0\le i\le n-1$). Then, the number of edges crossing the cut $(S,\overline{S})$ is given by

\[\begin{aligned} \text{objective} &= \sum_{(i,j)\in E}\Bigl(x_i(1-x_j) + (1-x_i)x_j\Bigr). \end{aligned}\]

Since the QUBO problems aims to minimize an objective function, we obtain a QUBO expression $f$ by negating the objective:

\[\begin{aligned} f &= -\,\text{objective}. \end{aligned}\]

An optimal assignment minimizing $f$ corresponds to a maximum cut of $G$.

PyQBPP program for the Max-Cut problem

import pyqbpp as qbpp

N = 16
edges = [
    (0, 1),  (0, 2),  (1, 3),  (1, 4),  (2, 5),  (2, 6),  (3, 7),
    (3, 13), (4, 6),  (4, 7),  (4, 14), (5, 8),  (6, 8),  (6, 12),
    (6, 14), (7, 14), (8, 9),  (9, 10), (9, 12), (10, 11),(10, 12),
    (11, 13),(11, 15),(12, 14),(12, 15),(13, 15),(14, 15)]

x = qbpp.var("x", N)

objective = 0
for u, v in edges:
    objective += x[u] * ~x[v] + ~x[u] * x[v]

f = -objective
f.simplify_as_binary()

solver = qbpp.ExhaustiveSolver(f)
sol = solver.search()

print(f"objective = {sol(objective)}")
print("S:", end="")
for i in range(N):
    if sol(x[i]) == 1:
        print(f" {i}", end="")
print()

This program creates the expressions objective and f, where f is the negation of objective. The Exhaustive Solver minimizes f, and an optimal assignment is stored in sol.

This program prints the following output:

objective = 22

Visualization using matplotlib

The following code visualizes the Max-Cut solution using matplotlib and networkx:

import matplotlib.pyplot as plt
import networkx as nx

G = nx.Graph()
G.add_nodes_from(range(N))
G.add_edges_from(edges)
pos = nx.spring_layout(G, seed=42)

colors = ["#e74c3c" if sol(x[i]) == 1 else "#3498db" for i in range(N)]
edge_colors = ["#e74c3c" if sol(x[u]) != sol(x[v]) else "#cccccc"
               for u, v in edges]
edge_widths = [2.5 if sol(x[u]) != sol(x[v]) else 1.0
               for u, v in edges]
nx.draw(G, pos, with_labels=True, node_color=colors, node_size=400,
        font_size=9, edge_color=edge_colors, width=edge_widths)
plt.title("Max-Cut")
plt.savefig("maxcut.png", dpi=150, bbox_inches="tight")
plt.show()

The two partitions are shown in red and blue. Cut edges (crossing the partition) are highlighted in red.