OFFSET
1,2
EXAMPLE
a(2)=16 because there are 16 squarefree odd semiprimes < 10^2: 15=3*5, 21=3*7, 33=3*11, 35=5*7, 39=3*13, 51=3*17, 55=5*11, 57=3*19, 65=5*13, 69=3*23, 77=7*11, 85=5*17, 87=3*29, 91=7*13, 93=3*31, 95=5*19.
MATHEMATICA
-1 + Accumulate@ Table[Count[Range[10^n + 1, 10^(n + 1) - 1, 2], _?(And[ SquareFreeQ@ #, PrimeNu[#] == 2] &)], {n, 0, 5}] (* Michael De Vlieger, Oct 10 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, Oct 10 2017
EXTENSIONS
a(21) from Jinyuan Wang, Jul 30 2021
STATUS
approved