1861

This number is a prime.

+ The American Civil War (also known as the War Between the States) started in 1861. Did you know there are exactly 18 primes less than or equal to 61? [Greer]

+ The smallest prime formed from the sum of the squares of the first n not prime-free numbers, in this case, for n = 8 we have: 4^2 + 8^2 + 9^2 + 12^2 + 16^2 + 18^2 + 20^2 + 24^2 = 1861. [Post]

+ The smallest prime whose the reversal is a concatenation of two composite squares. [Loungrides]

+ The sum of the squares of two consecutive integers (30^2 + 31^2 = 1861). [Francois]

+ Archaeopteryx was named from a single feather in 1861. [Shelton]

+ There are no known (probable) primes of the form 186{1} (i.e., 186111...111) other than 1861. [Luen]

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