OFFSET
0,4
COMMENTS
EXAMPLE
a(0) = 1 because 2 is prime, 2 + 2 = 4 semiprime and 2 - 10^0 = 1,
a(1) = 1 because 11 and 13 are twin primes and 11 - 10^1 = 1,
a(2) = 1 because 101 and 103 are twin primes and 101 - 10^2 = 1,
a(3) = 9 because 1009 is prime, 1011 = 3*337 semiprime and 1009 - 10^3 = 9,
a(4) = 7 because 10007 and 10009 are twin primes and 10007 - 10^4 = 7,
a(5) = 19 because 100019 is prime, 100021 = 29*3449 semiprime and 100019 - 10^5 = 19, etc.
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Nov 03 2006
STATUS
approved