Monday, May 23

577, 5569, ...

This incipient-sequence comment in T.D. Noe's A138290 recently caught my eye. So, prime p such that 2^(p+1)-2^q-1 is composite for all positive q < p. Carlos Rivera had it published as Puzzle 437 in 2008. Therein, Giovanni Resta notes that he had checked up to 15373 without finding a third term. All of my processors were busy of course but I hadn't been using Catherine's computer in a while (too slow for my needs), so...

This morning (after several weeks of number crunching) I noticed that her machine had come up with the third term: 29251. To give you an idea of the computing involved: on my latest iMac, p = 577 (174-digit numbers) verified compositeness for all terms immediately; p = 5569 (1677-digit numbers) took a minute; and p = 29251 (8806-digit numbers) needed more than four hours!

[The sequence became A278740 on 27 Nov 2016.]

No comments:

Post a Comment