Take the numbers from 82 stepping down to one, and write them all out together. You get a very big number.
828180797877767574737271706968676665646362616059585756555453525150494847464544\\ 43424140393837363534333231302928272625242322212019181716151413121110987654321
It’s prime. What’s slightly odd is that there are no other numbers constructed the same way for n < 500 which are also prime.
[tags]Mathematics,Prime Numbers[/tags]
Addendum: I got the number from Prime Numbers by David Wells. I wrote a little Python script that did the Rabin-Miller primality test, and first verified that the number specified above was (at least very likely to be) prime, and then generated all similar numbers up to 500, and found they were all composite.
Addendum2: In 1964, Gillies discovered the 21st, 22nd and 23rd Mersenne primes using the Illiac II supercomputer. It took them 2 hours and 15 minutes to verify that 2**11213-1 is prime. I implemented the Lucas Lehmer test in Python, and verified the result in 40 seconds on my AMD64. I love Moore’s Law.