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Russian Multiplication

Also called Ethiopian Multiplication. To multiply two numbers $a$ and $b$, write $a_0\equiv a$ and $b_0\equiv b$ in two columns. Under $a_0$, write $\left\lfloor{a_0/2}\right\rfloor $, where $\left\lfloor{x}\right\rfloor $ is the Floor Function, and under $b_0$, write $2b_0$. Continue until $a_i=1$. Then cross out any entries in the $b$ column which are opposite an Even Number in the $a$ column and add the $b$ column. The result is the desired product. For example, for $a=27, b=35$

\matrix{\hfill 27\cr\hfill 13\cr\hfill 6\cr\hfill 3\cr\hfill...
\cr\hfill 280\cr\hfill \underline {560}\cr
\hfill 945\cr}

© 1996-9 Eric W. Weisstein