OFFSET
1,1
COMMENTS
The given expression seems to produce safe primes quite frequently. The numbers 6*(4620*a(n)-1) form a subsequence of A230032.
Note that 4620 = 2^2*3*5*7*11.
Is a(4135..4139) = 59497..59501 the only set of 5 terms with common difference = 1? Checked for all terms up to 10^7. - Zak Seidov, Nov 02 2013
This is common because 4620 = 12*5*7*11 contains several small prime factors, which reduces the possible 12x-1 remainders, forcing all safe primes to be in other 12x-1 remainders mod 4620. Something similar would happen if 4620 were replaced with 12*5*7*13 = 5460. - Mark Andreas, Dec 31 2021
LINKS
Jon E. Schoenfield, Table of n, a(n) for n = 1..10000
PROG
(PARI) {f=4*3*5*7*11; for(k=1, 2000, isprime(p=k*f-1)&&isprime(p\2)&&print1(k", "))}
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, Oct 31 2013
STATUS
approved