Euler's identity
Alternate timeline
(covers information from an alternate timeline)
Seven's writing of Euler's identity
Euler's identity is the mathematical equation eiπ + 1 = 0, named after 18th century Earth mathematician Leonhard Euler.
Disoriented after awakening in an alternate timeline 2401, Seven of Nine wrote the equation on a mirror with lipstick as part of a self-test of her cognitive function. (PIC: "Penance")