About the Riesel Problem
Hans Ivar Riesel (May 28, 1929 in Stockholm  December 21, 2014) was a Swedish mathematician. In 1956, he showed that there are an infinite number of positive odd integer k's such that k*2^n1 is composite (not prime) for every integer n>=1. These numbers are now called Riesel numbers. He further showed that k=509203 was such one.
It is conjectured that 509203 is the smallest Riesel number. The Riesel problem consists in determining that 509203 is the smallest Riesel number. To show that it is the smallest, a prime of the form k*2^n1 must be found for each of the positive integer k's less than 509203. As of October 4th, 2014, there remain 50 k's for which no primes have been found. They are as follows:
2293, 9221, 23669, 31859, 38473, 46663, 67117, 74699, 81041, 93839, 97139, 107347, 121889, 129007, 143047, 146561, 161669, 192971, 206039, 206231, 215443, 226153, 234343, 245561, 250027, 273809, 315929, 319511, 324011, 325123, 327671, 336839, 342847, 344759, 362609, 363343, 364903, 365159, 368411, 371893, 384539, 386801, 397027, 409753, 444637, 470173, 474491, 477583, 485557, 494743
For a more detailed history and status of the Riesel problem, please visit Wilfrid Keller's The Riesel Problem: Definition and Status.
To participate in effort, please select The Riesel Problem (LLR) and/or The Riesel Problem (Sieve) projects in your PrimeGrid preferences section.
Additional Information
The Riesel problem is to k*2^n1 as the Sierpinski problem is to k*2^n+1. There is no equivalent to the 'prime' Sierpinski problem since k=509203, the conjectured smallest Riesel number, is prime.
Primes found at PrimeGrid
191249*2^34176961 by Jonathan Pritchard on 21 Nov 2010. Official Announcement
428639*2^35064521 by Brett Melvold on 14 Jan 2011. Official Announcement
65531*2^36293421 by Adrian Schori on 5 Apr 2011. Official Announcement
123547*2^38048091 by Jakub Ćuszczek on 8 May 2011. Official Announcement
415267*2^37719291 by Alexey Tarasov on 8 May 2011. Official Announcement
141941*2^42994381 by Scott Brown on 26 May 2011. Official Announcement
353159*2^43311161 by Jaakko Reinman on 31 May 2011. Official Announcement
162941*2^9937181 by Dmitry Domanov on 02 February 2012. Official Announcement
252191*2^54978781 by Jan Haller on 23 June 2012. Official Announcement.
398023*2^64180591 by Vladimir Volynsky on 5 October 2013. Official Announcement.
304207*2^66435651 by Randy Ready on 10 October 2013. Official Announcement.
40597*2^68085091 by Frank Meador on 25 December 2013. Official Announcement.
402539*2^71730241 by Walter Darimont on 2 October 2014. Official Announcement.
502573*2^71819871 by Denis Iakovlev on 4 October 2014. Official Announcement.
