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73 — 'best number' per Sheldon Cooper
In The Big Bang Theory, Sheldon Cooper explains why 73 is the best number: it's prime, the 21st prime (7×3=21), its mirror 37 is also prime and the 12th (1+2=3, and √9=3). Fans made 73 a cult number.
Culture
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73 — prime with unique properties
73 is the 21st prime number. It is a so-called 'Sheldon prime' — a prime number whose rank (21) equals the product of its digits (7×3). Such numbers are extremely rare.
Numerology
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Ham radio — 73 means 'best regards'
In Q code and ham radio traditions, '73' means 'best regards'. This code has been used since the 19th century and remains the standard farewell in radio communications worldwide.
History