in oblique cases, noun and number take both this case, except that the numbers ending with "thousand", "million", "billion" etc. (nouns: (f.), (m.), (m.)) in singular or in plural are regarded as nouns and always require genitive case in plural: (instr.) (gen.); cf. and (all instr.). initial (leftmost) numeral "1" can be omitted in combinations ( – 1001 nights), , etc.