Weierstrass function

The first published function that is continuous but has no slope at any point. It sums cosine waves whose frequency multiplies by b while their height shrinks by a. Here each wave drifts at its own pace, so the jagged line ripples.

W(x) = Σ aⁿ cos(bⁿ π x)

Courbe de Weierstrass: mathcurve lists it without an entry, so the link goes to Wikipédia