I wouldn't call sum(something) a formula, but a calculating
instruction. For example pi(x) = sum(1)(2,3,5
x) looks great, but is
only an instruction to count primes up to x, while the formula is
pi(x) ~ x/ln x. Thus pi(2)(x) = sum (pi(x/p i)-i+1) (i=1
looks very spectacular, but is only the description of counting the
semiprimes up to x. The formula for it might be
pi(2)(x) ~ n/(e ln ln x) or the like, perhaps someone knows a better
--- In email@example.com
, "Werner D. Sand"
> Perfect answer! Thank you, Mike.
> --- In firstname.lastname@example.org, mikeoakes2@ wrote:
> > An integer with exactly k prime factors is somethimes called a
> > k-almost-prime.
> > Here's a link:-
> > http://mathworld.wolfram.com/AlmostPrime.html
> > -Mike Oakes
> > -----Original Message-----
> > From: Theo.3.1415@
> > To: email@example.com
> > Sent: Wed, 31 Jan 2007 9.13AM
> > Subject: [PrimeNumbers] Two prime factors formula?
> > Does anyone know an approximate formula (according to x/ln x) for
> > number of positive integers up to x which are the product of
> exactly 2
> > prime factors: 4,6,9,10,14,15,21,22â¦? (NOT: 2 different prime
> > as in 12, 36,225â¦). Perhaps a general formula for n prime