In probability theory, the formula for the Poisson distribution is
Pm(n) = mne–m/n!
Pleasingly, the mnemonic for this is mnemonic: “m to the n, e to the –m over n factorial.” Arguably the factorial sign even resembles an inverted i.
Now we just need a way to remember that …
(From M.H. Greenblatt, Mathematical Entertainments, 1965.)