Não informado. (1921). Bulletin of the Astronomical Institutes of the Netherlands. Astronomical Institutes of the Netherlands.
শিকাগো স্টাইল (17 তম সংস্করণ) উদ্ধৃতিNão informado. Bulletin of the Astronomical Institutes of the Netherlands. Amsterdam: Astronomical Institutes of the Netherlands, 1921.
M.L.A (8 ম সংস্করণ) উদ্ধৃতিNão informado. Bulletin of the Astronomical Institutes of the Netherlands. Astronomical Institutes of the Netherlands, 1921.
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