OFFSET
0,2
COMMENTS
3-almostprime equivalent of Wilson's primeth recurrence: A007097.
MATHEMATICA
ThreeAlmostPrimePi[n_] := Sum[PrimePi[n/(Prime@i*Prime@j)] - j + 1, {i, PrimePi[n^(1/3)]}, {j, i, PrimePi@Sqrt[n/Prime@i]}]; ThreeAlmostPrime[n_] := Block[{e = Floor[Log[2, n] + 1], a, b}, a = 2^e; Do[b = 2^p; While[ThreeAlmostPrimePi[a] < n, a = a + b]; a = a - b/2, {p, e, 0, -1}]; a + b/2]; NestList[ThreeAlmostPrime@# &, 1, 18]
CROSSREFS
KEYWORD
nonn
AUTHOR
Jonathan Vos Post and Robert G. Wilson v, May 31 2006
EXTENSIONS
a(19)-a(20) from Donovan Johnson, Sep 29 2010
STATUS
approved