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 ≤ 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]

The only prime below 10^14 that is a sum of two consecutive squares (1861 = 30^2 + 31^2), where the reversal is the square of a prime (1681 = 41^2). The root 41 is itself a prime that is a sum of two consecutive squares 4^2 + 5^2. [Adams]