0,2

Apart from the first nonzero term the sequence is identical to A036352. - Hugo Pfoertner (hugo(AT)pfoertner.org), Jul 22 2003

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

Index entries for sequences related to numbers of primes in various ranges

(1/2)*( Pi(10^(n/2)) + Sum_{i=1..Pi(10^n)} Pi( (10^n-1)/P_i) ) = Sum_{i=1..Pi(sqrt(10^n))} Pi( (10^n-1)/P_i ) - Binomial( Pi(sqrt(10^n)), 2) (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005)

Below 10 there are three semiprimes: 4 (2*2), 6 (2*3) and 9 (3*3).

f[n_] := Sum[ PrimePi[(10^n - 1)/Prime[i]], {i, PrimePi[ Sqrt[10^n]]}] - Binomial[ PrimePi[ Sqrt[10^n]], 2]; Do[ Print[ f[n]], {n, 0, 14}] (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005)

Cf. A001358, A064911, A072000.

Cf. A036352 (identical starting from a(2)).

Adjacent sequences: A066262 A066263 A066264 this_sequence A066266 A066267 A066268

Sequence in context: A141789 A121077 A024396 this_sequence A134491 A045727 A105713

nonn

Patrick De Geest (pdg(AT)worldofnumbers.com), Dec 10 2001.

More terms from Hugo Pfoertner (hugo(AT)pfoertner.org), Jul 22 2003

a(14) from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005