Não informado. (1921). Bulletin of the Astronomical Institutes of the Netherlands. Astronomical Institutes of the Netherlands.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रNão informado. Bulletin of the Astronomical Institutes of the Netherlands. Amsterdam: Astronomical Institutes of the Netherlands, 1921.
एमएलए (8वां संस्करण) प्रशस्ति पत्रNão informado. Bulletin of the Astronomical Institutes of the Netherlands. Astronomical Institutes of the Netherlands, 1921.
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