OFFSET
1,3
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..63 (using data from A120033, terms n=48, 50..57 from Dana Jacobsen)
Eric Weisstein's World of Mathematics, Semiprime.
FORMULA
a(n) = A072000(2^n). - R. J. Mathar, Aug 26 2011
MATHEMATICA
SemiPrimePi[n_] := Sum[ PrimePi[ n/Prime@i] - i + 1, {i, PrimePi@ Sqrt@n}]; Table[ SemiPrimePi[2^n], {n, 47}]
PROG
(PARI) a(n)=my(s, i, N=2^n); forprime(p=2, sqrtint(N), s+=primepi(N\p); i++); s - i * (i-1)/2 \\ Charles R Greathouse IV, May 12 2013
(Perl) use ntheory ":all"; print "$_ ", semiprime_count(1 << $_), "\n" for 1..48; # Dana Jacobsen, Sep 10 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Dec 29 2006
STATUS
approved