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The number 81 has several fascinating properties and cultural significances. Mathematically, it is a perfect square (9² = 81) and also a perfect fourth power (3⁴ = 81). It is the product of the primes 3 and 27 (3 × 27). In base 10, it is the last two-digit number where all digits are perfect squares (8 and 1). In science, 81 is the atomic number of thallium, a metal used in electronics, glass manufacturing, and previously in medical imaging. Culturally, in many East Asian traditions, 81 is considered a lucky number because it is composed of 9 (a number associated with longevity and eternity) squared. Additionally, in history, the year 81 AD marked the beginning of the reign of Roman Emperor Domitian, who significantly impacted the Roman Empire's development.