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[2]
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[3]
GREENSTEIN, 1948, AP J, V107, P376
[4]
HUBBLE, 1922, AP J, V56, P404
[5]
HUBBLE, 1922, AP J, V56, P181
[8]
1953, AP J, V118, P318
[9]
1952, AP J, V116, P251