OFFSET
0,5
EXAMPLE
There are 7 eleven-almost primes up to 10000: 2048, 3072, 4608, 5120, 6912, 7168 & 7680.
MATHEMATICA
AlmostPrimePi[k_Integer, n_] := Module[{a, i}, a[0] = 1; If[k == 1, PrimePi[n], Sum[PrimePi[n/Times @@ Prime[Array[a, k - 1]]] - a[k - 1] + 1, Evaluate[ Sequence @@ Table[{a[i], a[i - 1], PrimePi[(n/Times @@ Prime[Array[a, i - 1]])^(1/(k - i + 1))]}, {i, k - 1}]]]]]; (* Eric W. Weisstein, Feb 07 2006 *)
Table[AlmostPrimePi[11, 10^n], {n, 12}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Feb 07 2006
EXTENSIONS
a(14) from Robert G. Wilson v, Jan 07 2007
STATUS
approved