This is a 10 Turkish lira banknote featuring Cahit Arf.

Printed next to his portrait is the Arf invariant:
Arf(q) = Σ q(aᵢ)q(bᵢ) ∈ ℤ₂

Here, (q) is a quadratic form over ( \mathbb{F}_2 ), and ((a_i, b_i)) is a symplectic basis.
The remarkable fact: this single bit (0 or 1) completely classifies non-degenerate quadratic forms over ( \mathbb{F}_2 ) up to equivalence.

Abstract algebra, compressed into a banknote.