[1]:

import graspologic

import numpy as np
import matplotlib.pyplot as plt


Consider the scenario where one would like to match graphs $$A$$ and $$B$$ with $$n_1$$ and $$n_2$$ nodes, respectively, where $$n_1 < n_2$$. The most straightforward fashion to ‘pad’ $$A$$, such that $$A$$ and $$B$$ have the same shape, is to add $$n_2 - n_1$$ isolated nodes to $$A$$ (represented as empty row/columns in the adjacency matrix). This padding scheme is known as $$\textit{naive padding}$$, substituting $$A \oplus 0_{(n_2-n_1)x(n_2-n_1)}$$ and $$B$$ in place of $$A$$ and $$B$$, respectively.

The effect of this is that one matches $$A$$ to the best subgraph of $$B$$. That is, the isolated vertices added to $$A$$ through padding have an affinity to the low-density subgraphs of $$B$$, in effect giving the isolates a false signal.

Instead, we may desire to match $$A$$ to the best fitting induced subgraph of $$B$$. This padding scheme is known as $$\textit{adopted padding}$$, and is achieved by substituting $$\tilde{A} \oplus 0_{(n_2-n_1)x(n_2-n_1)}$$ and $$\tilde{B}$$ in place of $$A$$ and $$B$$, respectively, where $$\tilde{A} = 2A - 1_{n_1}1_{n_1}^T$$ and $$\tilde{B} = 2B - 1_{n_2}1_{n_2}^T$$.

To demonstrate the difference between the two padding schemes, we sample two graph’s $$G_1'$$ and $$G_2$$, each having 400 vertices, from a $$0.5 \sim SBM(4,b,\Lambda)$$, where b assigns 100 vertices to each of the k = 4 blocks, and

\begin{align*} \Lambda &= \begin{bmatrix} 0.9 & 0.4 & 0.3 & 0.2\\ 0.4 & 0.9 & 0.4 & 0.3\\ 0.3 & 0.4 & 0.9 & 0.4\\ 0.2 & 0.3 & 0.4 & 0.7 \end{bmatrix}\\ \end{align*}

We realize $$G_1$$ from $$G_1'$$ by removing 25 nodes from each block of $$G_1'$$, yielding a 300 node graph (example adapted from section 2.5 of [1]).

The goal of the matching in this case is to recover $$G_1$$ by matching the right most figure below and $$G_2$$. That is, we seek to recover the shared community structure common between two graphs of differing shapes.

[1] D. Fishkind, S. Adali, H. Patsolic, L. Meng, D. Singh, V. Lyzinski, C. Priebe, “Seeded graph matching”, Pattern Recognit. 87 (2019) 203–215

## SBM correlated graph pairs¶

[2]:

# simulating G1', G2, deleting 25 vertices
from graspologic.match import GraphMatch as GMP
from graspologic.simulations import sbm_corr
from graspologic.plot import heatmap
np.random.seed(1)

directed = False
loops = False
block_probs = [[0.9,0.4,0.3,0.2],
[0.4,0.9,0.4,0.3],
[0.3,0.4,0.9,0.4],
[0.2,0.3,0.4,0.7]]
n =100
n_blocks = 4
rho = 0.5
block_members = np.array(n_blocks * [n])
n_verts = block_members.sum()
G1p, G2 = sbm_corr(block_members,block_probs, rho, directed, loops)
G1 = np.zeros((300,300))
c = np.copy(G1p)

step1 = np.arange(4) * 100 + 75
step2 = np.arange(5) * 75
step3 = np.arange(4) * 100
for i in range(len(step1)):
block1 = np.arange(step1[i], step1[i]+25)
c[block1,:] = -1
c[:, block1] = -1
for j in range(len(step3)):
G1[step2[i]:step2[i+1], step2[j]:step2[j+1]] = G1p[step3[i]: step1[i], step3[j]:step1[j]]

topleft_G1 = np.zeros((400,400))
topleft_G1[:300,:300] = G1
fig, axs = plt.subplots(1, 4, figsize=(20, 10))
heatmap(G1p, ax=axs[0], cbar=False, title="G1'")
heatmap(G2, ax=axs[1], cbar=False, title="G2")
heatmap(c, ax=axs[2], cbar=False, title="G1")
heatmap(topleft_G1, ax=axs[3], cbar=False, title="G1 (to top left corner)")

[2]:

<AxesSubplot:title={'center':'G1 (to top left corner)'}>


[3]:

np.random.seed(1)

seed1 = np.random.choice(np.arange(300),8)
seed2 = [int(x/75)*25 + x for x in seed1]
gmp_naive = gmp_naive.fit(G2, G1, seed2, seed1)
G1_naive = topleft_G1[gmp_naive.perm_inds_][:, gmp_naive.perm_inds_]

fig, axs = plt.subplots(1, 2, figsize=(14, 7))

naive_matching = np.concatenate([gmp_naive.perm_inds_[x * 100 : (x * 100) + 75] for x in range(n_blocks)])
adopted_matching = np.concatenate([gmp_adopted.perm_inds_[x * 100 : (x * 100) + 75] for x in range(n_blocks)])

print(f'Match ratio of nodes remaining in G1, with naive padding: {sum(naive_matching == np.arange(300))/300}')

Match ratio of nodes remaining in G1, with naive padding: 0.09333333333333334

We observe that the two padding schemes perform as expected. The naive scheme permutes $$G_1$$ such that it matches a subgraph of $$G_2$$, specifically the subgraph of the first three blocks. Additionally, (almost) all isolated vertices of $$G_1$$ are permuted to the fourth block of $$G_2$$.
On the other hand, we see that adopted padding preserves the common block structure between $$G_1$$ and $$G_2$$.
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