# 1811

This number is a prime.

The year self-educated Peter Barlow published *An Elementary Investigation of the Theory of Numbers*. He commented that 2^{30}(2^{31} - 1) is the greatest perfect number that will ever be discovered, for, as they are merely curious without being useful, it is not likely that any person will attempt to find one beyond it. Quoted in *Conic Sections* (Dublin 1993).

The product of 1811 and the next two consecutive primes results in a concatenation of two consecutive primes in descending order. [De Geest]

Chang and Eng, the most famous set of "Siamese twins," were born in Siam (modern day Thailand) in 1811.

The year Italian physicist and lawyer Amedeo Avogrado first determined the volume of one mole of a gas. [Tittle]

The number of Officer Murphy's mailbox in the movie, "Robocop 2."

The French mathematician Évariste Galois was born in 1811. [Beedassy]

The smallest invertible prime p such that replacing each digit d in decimal expansion of p with d^2 yields a new prime, a semiprime and a sphenic number when done recursively three times, i.e., 1811-> 16411 (prime); 16411->1361611 (semiprime); 1361611->193613611 (sphenic number). [Bajpai]

An invertible prime formed from the concatenation of the first n positive strobogrammatic numbers, i.e., 1, 8, and 11. Can you find the next one? [Honaker]

The Saros cycle is approximately 18 years, 11 days long. [NASA]