1,1

The n-th row's sum is 10^n - 1.

Robert G. Wilson v, Table of n, a(n) for n = 1..342

Eric Weisstein's World of Mathematics, Semiprime.

Eric Weisstein's World of Mathematics, Almost Prime.

4 4 1

25 34 22 12 4 2

168 299 247 149 76 37 14

7 2

1229 2625 2569 1712 963 485 231

105 47 22 7 3 1

9592 23378 25556 18744 11185 5933 2973

1418 671 306 138 63 25 11 4 2

78498 210035 250853 198062 124465 68963 35585 17572

8491 4016 1878 865 400 179 79 35 14 7 2

664579 1904324 2444359 2050696 1349779 774078 409849 207207

101787 49163 23448 11068 5210 2406 1124 510 233 102 45 21 7 3 1

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[m, 10^n], {n, 6}, {m, Floor[Log[2, 10^n]] }] // Flatten

By columns: A006880, A036352, A109251, A114106, A114453, A120047, A120048, A120049, A120050, A120051, A120052, A120053.

The n-th row's sum: A002283 = 10^n -1, A116430, A126279: same array but for powers of two.

Sequence in context: A079507 A098364 A116866 * A170986 A071207 A136214

Adjacent sequences: A126277 A126278 A126279 * A126281 A126282 A126283

tabf,less,nonn

Jonathan Vos Post & Robert G. Wilson v, Dec 22 2006

approved