Addendum to $e^{i \pi}$
Benjamin Peirce apparently liked this mathematical synonym for the additive inverse of $1$ so much that he introduced three special symbols for $e, i, \pi$ — ones that enable $e^{i \pi}$ to be written in a single cursive ligature, like so:

- Benjamin Peirce (1870/1882), Linear Associative Algebra, § 15, p. 5.


