OFFSET
0,2
COMMENTS
See also A109611 (Chen primes: primes p such that p + 2 is either a prime or a semiprime), A102540 (primes that are not Chen primes). a(n) is prime for a(1) = 43, a(5) = 353 [Chen prime], a(7) = 607, a(15) = 2377, a(31) = 9049, a(35) = 11489 [Chen prime], a(49) = 22013. a(n) is semiprime for a(3) = 177, a(9) = 943, a(13) = 1829, a(17) = 2973, a(19) = 3653, a(21) = 4393, a(23) = 5173, a(24) = 5594, a(29) = 7909, a(37) = 12793, a(38) = 13466, a(40) = 14866, a(41) = 15593, a(43) = 17065, a(45) = 18595.
FORMULA
a(n) = SUM[k=1..n] A102540(k).
EXAMPLE
a(5) = 43 + 61 + 73 + 79 + 97 = 353, which happens to be the Chen prime A109611(52).
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Mar 09 2006
STATUS
approved