Sheffer stroke
A Sheffer stroke is a single binary operator that is sufficient to express every operation in a formal system. "Functional completeness from one primitive."
The classic example: NAND
In Boolean logic:
NAND(a, b) = ¬(a ∧ b)
Every Boolean function, AND, OR, NOT, XOR, IMPLY, can be written as a nested composition of NANDs. This is what makes NAND the "workhorse" of digital logic. NOR has the same property.
Named after Henry M. Sheffer
Sheffer showed in 1913 that a single binary operator (what we now call the Sheffer stroke) suffices for propositional logic. It's still the foundational result for minimal-basis questions.
The continuous analog
Until 2026, continuous mathematics had no known Sheffer stroke, no single binary operator from which you could reconstruct every elementary function. Odrzywołek's paper presents eml as such an operator (with one constant, 1).