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 A116426 The number of n-almost primes less than or equal to 4^n, starting with a(0)=1. 8
 1, 2, 6, 13, 34, 77, 177, 406, 887, 1962, 4225, 9094, 19482, 41414, 87706, 184976, 389357, 816193, 1708412, 3566209, 7431153, 15457234, 32098652, 66560309, 137830562, 285062028, 588871107, 1215176367, 2505048537, 5159228725 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS 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[n, 4^n], {n, 29}] CROSSREFS Cf. A078840, A078841, A078842, A116432, A078843, A116426, A078844, A116427, A078845, A116428, A116429, A116430, A078846, A116431. Sequence in context: A238827 A098407 A151390 * A196906 A162057 A026550 Adjacent sequences:  A116423 A116424 A116425 * A116427 A116428 A116429 KEYWORD nonn AUTHOR Robert G. Wilson v, Feb 10 2006 STATUS approved

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Last modified March 23 17:19 EDT 2019. Contains 321432 sequences. (Running on oeis4.)