OFFSET
1,1
COMMENTS
Likely the union of the Mersenne primes A000668 and the {8}. [Presence of Mersenne primes M is obvious because A085392(M+1)=2 and A085392(M)=1.
Absence of other primes p is also clear because A085392(p)=1 and A085392(p+1) >=3 because it contains at least one odd prime factor if not of the Mersenne type.
For composite candidates c, we search adjacent c+1 and c with largest noncomposite divisors 2 and 1 or 3 and 2. The first branch enforces c=2 which is immediately discarded. The second branch searches for a power of 3 adjacent to a power of 2, and the solution to this exponential diophantine equation 3^x-2^y=1 is believed to lead only to the 8 (see the Weger review in the link).]
LINKS
B. M. M. de Weger, Book review, Bull. Am. Math. Soc. 25 (1991) 145-146.
EXAMPLE
PROG
(PARI) gpd(n) = if (n==1, 1, n/factor(n)[1, 1]);
gpf(n) = if (n==1, 1, vecmax(factor(n)[, 1]));
f(n) = gpf(gpd(n));
isok(n) = f(n+1) - f(n) == 1; \\ Michel Marcus, Apr 08 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Jun 09 2011
STATUS
approved