<｜begin▁of▁sentence｜>### Instruction:
As an astrophysics researcher, you have a graph representing collaborations within the astrophysics community in the 'astro-ph.gml' file. You are interested in identifying communities of researchers within this network, with each community consisting of at least 20 nodes but no more than 50 nodes. You also want to use predefined seed nodes to guide the community detection process. Finally, you would like to represent the resulting node clustering in a JSON format for further analysis.
### Response:
import networkx as nx
from cdlib import algorithms

# Load the Astrophysics collaborations graph
G = nx.read_gml('astro-ph.gml')

# Seed nodes for lemon algorithm
seed_nodes = [1, 3, 7]  # These would be specific to the graph structure and problem domain

# Perform community detection using LEMON
communities = algorithms.lemon(G, seeds=seed_nodes, min_com_size=20, max_com_size=50)

# Get a JSON representation of the community structure
communities_json = communities.to_json()
print(communities_json)
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