This number is a prime.
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During the Tokugawa Period in Japan, the long road from Kyoto to Edo (now Tokyo), had 53 rest stops along the way. [Haga]
The prime 53 figured in the 1945 WWII battle for Iwo Jima. Task Force 53 brought the 71000 soldiers and 41000 marines. The officer commanding the Japanese troops was Lt. Gen. Tadamichi Kuribayashi, age 53. The average height of a Japanese infantryman was 5'3". Source: History TV Channel. [Haga]