This generalizes to multiplying any number times 11, https://en.wikipedia.org/wiki/Trachtenberg_system#Multiplying_by_11 https://en.wikipedia.org/wiki/Trachten

This generalizes to multiplying any number times 11, https://en.wikipedia.org/wiki/Trachtenberg_system#Multiplying_by_11 https://en.wikipedia.org/wiki/Trachtenberg_system#Multiplyin... Take a number, say 142857. Prepend a 0 on the left. Underneath each digit write the sum of the digit above and the one to the right. If you work from the right, it's easier to keep track of the carries. 0142857 1571427