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1 Introduction
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2 The ICRF and its two extensions, ICRF-Ext.1 and ICRF-Ext.2
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3 Quasar catalogs at optical wavelengths
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4 Cross-identification between ICRF and catalogs of quasars
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5 Photometric properties of the ICRF quasars
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6 About the sky coverage of the ICRF quasars and other catalogs
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7 Conclusion
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References
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