Shadowing lemma

Given a map f : X → X of a metric space (X, d) to itself, define a ε-pseudo-orbit (or ε-orbit) as a sequence ${\displaystyle (x_{n})}$ of points such that ${\displaystyle x_{n+1}}$ belongs to a ε-neighborhood of ${\displaystyle f(x_{n})}$.

Then, near a hyperbolic invariant set, the following statement holds:^[4] Let Λ be a hyperbolic invariant set of a diffeomorphism f. There exists a neighborhood U of Λ with the following property: for any δ > 0 there exists ε > 0, such that any (finite or infinite) ε-pseudo-orbit that stays in U also stays in a δ-neighborhood of some true orbit.

${\displaystyle \forall (x_{n}),\,x_{n}\in U,\,d(x_{n+1},f(x_{n}))<\varepsilon \quad \exists (y_{n}),\,\,y_{n+1}=f(y_{n}),\quad {\text{such that}}\,\,\forall n\,\,x_{n}\in U_{\delta }(y_{n}).}$

In Men at Arms (1993), a novel by Terry Pratchett, the "Shadowing Lemma" appears as a fictional creature that "exists in only two dimensions and eats mathematicians." In the same passage, Pratchett also references the "quantum weather butterfly," a nod to the butterfly effect and chaos theory.

• Chaotic systems
• Butterfly effect