Which mathematical concept has become a dominant brand positioning element in mathematics textbook cover designs and publisher logos in the past two decades?

In the competitive world of educational publishing, mathematics textbooks have undergone significant brand positioning shifts over the decades. One particular design element has become increasingly prominent in mathematics textbook covers and logos since the early 2000s, used to signal sophistication and mathematical elegance. This design choice reflects publishers' attempts to make mathematics appear more visually appealing and connected to nature and art.

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The Golden Ratio (φ ≈ 1.618) and Fibonacci spiral imagery
Pascal's Triangle patterns and symmetry
Mandelbrot fractal set visualizations
Klein bottle and Möbius strip topological forms