Não informado. Publications of the Kapteyn Astronomical Laboratory at Groningen. University of Groningen.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रNão informado. Publications of the Kapteyn Astronomical Laboratory at Groningen. Groningen, Holanda: University of Groningen.
एमएलए (8वां संस्करण) प्रशस्ति पत्रNão informado. Publications of the Kapteyn Astronomical Laboratory at Groningen. University of Groningen.
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