Casting Out 9s/Historical Note

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Historical Note on Casting Out 9s

The technique of Casting Out 9s was probably an invention of the mathematicians of early India.

It appears in Bhaskara II Acharya's Lilavati, dating from about $1150$.

It arrived in Western Europe from the Arabs, via the Liber Abaci of $1202$ by Leonardo Fibonacci.

When initially invented, the technique involved dividing the numbers involved by $9$ and adding the remainders, but the summing of the digital roots has the same effect.


Sources