I still recall my thrill and disappointment when I read Mathematical
Carnival [4], by Martin Gardner. I was thrilled because, as my
high school teacher had recommended, mathematics was presented in
a playful way that I had never seen before. I was disappointed because
it contained a formula that I thought I had “invented” a few years
earlier. I have always had a passion for mental calculation, and the
following formula appears in Gardner’s chapter on “Lightning Calculators.”
It was used by the mathematician A. C. Aitken to mentally
square large numbers.