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Arithmetic number
In historical documents, arithmetic numbers are mentioned. It is a vague concept, because it only says you can calculate with them.
Explanation
Usually positive numbers and the number zero are meant. Sometimes, however, it is just about the real numbers.
Example 1
Roots of negative numbers should be converted with the definition of the imaginary unit i2 = −1 . Then you don't have a minus sign anymore, and you can calculate the root. When multiplying, you get
It produces a negative number, and you probably wouldn't have expected that.