# Nomination via SGM¶

This class implements Vertex Nomination via Seeded Graph Matching (VNviaSGM) with the algorithm described in .

Given two graphs $$G_1$$ and $$G_2$$ with associated adjacency matrices $$A$$ and $$B$$, VNviaSGM proposes a nomination list of potential matches in graph $$G_2$$ to a vertex of interest $$voi \in G_1$$ with associated probabilities.

Let $$A_L(a)$$ be the induced subgraph derived from $$A$$, and centered about vertex $$a \in A$$ with a maximum distance from $$a$$ of $$L$$. VNviaSGM first finds $$A_L(voi)$$, and if no seeds are in this subgraph, the algorithm stops early and returns a nomination list of None.

Define $$S_A \subset A_L(voi)$$ to be the seed vertices in the subgraph centered around the voi, with associated seeds from graph $$B$$ ($$S_B$$).

Two subgraphs are then generated around $$S_A$$ for graph $$A$$, as well as around the associated seeds $$S_B$$ for graph $$B$$.

Specifically, define $$SG_1 = \underset{s_A \in S_A}{\bigcup} A_L(s_A)$$ and $$SG_2 = \underset{s_B \in S_B}{\bigcup} B_L(s_B)$$

These subgraphs ($$SG_1$$ and $$SG_2$$) are matched using SGM over several random initializations, resulting in probabilities corresponding to the proportion in which a node in $$B$$ is matching to the voi. See Graph Matching Algorithm Reference for more details.

 Patsolic, HG, Park, Y, Lyzinski, V, Priebe, CE. Vertex nomination via seeded graph matching. Stat Anal Data Min: The ASA Data Sci Journal. 2020; 13: 229– 244. https://doi.org/10.1002/sam.11454

:

from graspologic.nominate import VNviaSGM
from graspologic.simulations import er_np
from graspologic.plot import heatmap
import numpy as np
import matplotlib.pyplot as plt

np.set_printoptions(suppress=True)

:

# Define parameters
n = 50
p = 0.3
num_seeds = 4

voi = 5 # choose a vertex of interest

:

np.random.seed(2)
G1 = er_np(n=n, p=p)
node_shuffle_input = np.random.permutation(n)

G2 = G1[np.ix_(node_shuffle_input, node_shuffle_input)]

heatmap(G1, title = "Origional ER Graph (unshuffled)")
heatmap(G2, title = "Shuffled ER graph")

:

<AxesSubplot:title={'center':'Shuffled ER graph'}>  :

kklst= [(xx, yy) for xx, yy in zip(node_shuffle_input, np.arange(len(node_shuffle_input)))]
kklst.sort(key=lambda x:x)
print("Association voi in G1 to vertex in G2 =", kklst[voi])
kklst = np.array(kklst)

Association voi in G1 to vertex in G2 = (5, 37)


The algorithm produces an $$n \times 2$$ nomination list, where n is the number of nominees. Each row has the following format (vertex $$j \in G_2$$, probability that j matches voi). Note: the output is sorted with the largest probability coming first in the output list.

:

VNalg = VNviaSGM()
print(VNalg.fit_predict(G1, G2, voi, [kklst[0:num_seeds, 0], kklst[0:num_seeds, 1]]))

[[37.  1.]
[48.  0.]
[25.  0.]
[ 7.  0.]
[ 8.  0.]
[11.  0.]
[14.  0.]
[19.  0.]
[21.  0.]
[23.  0.]
[26.  0.]
[45.  0.]
[34.  0.]
[35.  0.]
[38.  0.]
[40.  0.]
[41.  0.]
[42.  0.]
[43.  0.]
[24.  0.]]


As seen, the actual correspondence is 5–37 and the model predicts that 5 (in graph $$G_1$$) matches with 37 (in graph $$G_2$$) with >90% confidence.