So there is a multiplication trick It says 9×13 say Half and double everything 4.5×26 Cancel out the .5 4×26 2×52 1×104 Cancel out the even left hindsight multipliers and add the left right hand numbers
So 9×13= 104+13 = 117 I know how this can be explained by working in binary .Is there a rigorous proof how this algorithm works in binary.