# 數系

Let $$\mathbb{N}$$ be the set of positive integers.

Natural numbers are closed under addition and multiplication.

The addition and multiplication of two natural numbers will yield a natural number.

Irrational numbers are not closed under addition, subtraction, multiplication, and division.

The positional system in base ten.

Roman numerals are not positional.

 [語音講解：numeral.mp3]

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Created: Sep 21, 2022
Last Revised: