19

This number is a prime.

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Khwārizmi (Father of Algebra) wrote several other works including a treatise on the Hebrew calendar, titled Risāla fi istikhrāj ta’rikh al-yahūd ("Extraction of the Jewish Era"). It describes the Metonic cycle, a 19-year intercalation cycle; the rules for determining on what day of the week the first day of the month Tishrei shall fall. [Nazeri]