OFFSET
1,1
COMMENTS
After the first element, 31000003, which is prime, integers in this sequence can never be prime, as they are all multiples of 3. They can be semiprimes, as is the case for 3671000367 = 3 x 1223666789, 4571000457 = 3 x 1523666819, 5411000541 = 3 x 1803666847, 9071000907 = 3 x 3023666969.
FORMULA
a(n) = Concatenate(P, P+1000000) iff P prime and P+1000000 prime.
EXAMPLE
Prime(47) = 211 and 211 + 1000000 = Prime(78515) = 1000211. Concatenating these two primes gives 2111000211 = 3^4 * 17^2 * 31 * 2909.
CROSSREFS
KEYWORD
base,easy,nonn
AUTHOR
Jonathan Vos Post, Mar 23 2005
STATUS
approved