This number is a prime.

+ A prime whose digits are consecutive nonprime digits reversed.

+ Pulsars (rapidly spinning neutron stars) have been observed to spin up to 641 times per second.

+ In 1732, Euler found that 2^2^5 + 1, the first composite Fermat number, is divisible by 641.

+ 641 divides all the Generalized Fermat numbers of the form (2^a*5^b)^2^5 + 1, where a is odd and b is even, or a is even and b is odd. [Gallot]

+ 641 = 42 + 252 = 24 + 54 = (12 + 12)4 + (12 + 22)4. [Melo]

+ 2641 - 1 was the smallest Mersenne number that had not been completely factored prior to the 21st century. [Cerias]

+ Replacing each digit d of 641 with d copies of the digit d produces another prime, i.e., 641 becomes 66666644441, which is also prime. If we apply the same transform using 66666644441, yet another prime is formed. [Honaker]

+ The Ford 641 Workmaster is an antique farm tractor. They have been known to crop up at the annual Lee County Tobacco & Fall Festival in Pennington Gap, Virginia (the birthplace of one of the authors).

+ 641 is the smallest prime factor of 4^16 + 1 , written with all the digits of 641 . [Capelle]

+ The largest sexy prime quadruplet less than a thousand starts at 641. I.e., 641+6, 641+6+6, and 641+6+6+6, are also prime. [Opao]

+ The leading 641 digits in the decimal expansion of the square root of three is prime. [Carmody]

+ 641 can be written as 10 * 26 + 1.

+ The Great Library of Alexandria, home to such mathematicians as Euclid, Archimedes, Eratosthenes, Apollonius, and Pappus, had disappeared before the arrival of the Muslim Arab armies in A.D. 641.

+ Room 641A is an alleged intercept facility operated by AT&T for the U.S. National Security Agency. [McCranie]

+ First of seven consecutive primes that remain that way if you eliminate their first digit. [Silva]

+ The largest 3-digit number n such that (10^n+1)/11 is prime. [Loungrides]

+ The only non-titanic prime of form 2^x+5^x, for x a composite number (x=4). [Loungrides]

+ The least number in the smallest set of four consecutive odd numbers that are primes or pseudoprime (base-2). [Gupta]

+ The sum of all the palindromes from 1 up to 101 is prime, i.e., 1+2+3+...+9+11+22+33+...+99+101 = 641. [Loungrides]

(There are 5 curios for this number that have not yet been approved by an editor.)

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