OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
87 is a semiprime and its digital root 6 is also a semiprime.
MATHEMATICA
Select[Range[680], PrimeOmega[#] == 2 && PrimeOmega[Mod[# - 1, 9] + 1] == 2 &] (* Amiram Eldar, Feb 06 2021 *)
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), May 20 2006
STATUS
approved