Which mathematical pricing strategy, based on a famous irrational number, is frequently used by math educational companies to psychologically optimize their product prices?

In the mathematics education industry, pricing strategies are crucial for textbooks, educational software, and online courses. The right pricing approach can significantly impact adoption rates and accessibility. This poll tests your knowledge about a famous mathematical pricing principle that has revolutionized how educational math products are valued and marketed. Can you identify the correct mathematical pricing strategy that has become standard in the industry?

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Phi Pricing (using the golden ratio 1.618... to create price points like $16.18 or $61.80)
Pi Pricing (setting prices at multiples of 3.14, such as $31.40 or $314)
Euler Pricing (using e = 2.718... to establish price points like $27.18 or $271.80)
Fibonacci Pricing (setting prices according to the Fibonacci sequence: $8, $13, $21, $34, etc.)