

A296653


a(n) is the smallest k > 15 such that the density of semiprimes in 1..k is 1/n.


0




OFFSET

3,1


COMMENTS

The condition that k > 15 is included in the definition because the ratio (number of semiprimes in 1..k)/k is 0 for k < 4 and reaches its maximum value (2/5) only at k = 10 (the 4th semiprime) and at k = 15 (the 6th semiprime), and decreases (although not monotonically) beyond that.


LINKS



FORMULA



EXAMPLE

For k > 15, the ratio (number of semiprimes in 1..k)/k first decreases to 
1/3 at k = 18 (the 6th semiprime), so a(3) = 18;
1/4 at k = 26432 (the 6608th semiprime), so a(4) = 26432.


CROSSREFS



KEYWORD

nonn,more


AUTHOR



EXTENSIONS



STATUS

approved



