C. K. Seyfert (1951). “A Dense Group of Galaxies in Serpens”. Publications of the Astronomical Society of the Pacific371: 72-75. Bibcode: 1951PASP...63...72S. doi:10.1086/126319.
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C. K. Seyfert (1951). “A Dense Group of Galaxies in Serpens”. Publications of the Astronomical Society of the Pacific371: 72-75. Bibcode: 1951PASP...63...72S. doi:10.1086/126319.