OFFSET
1,1
COMMENTS
Another good approximation to the number of twin primes < 10^n is the sum of twin primes < 10^(n/2)/4. For example Pi2(10^16) = 10304185697298.
SumPi2(10^8)/4 = 10301443659233 for an error of 0.0000266. However, the Hardy-Littlewood approximation is superior giving an error of -0.000000665.
FORMULA
C_2 = 0.660161815846869573927812110014555778432623. Li_2(x) = 2*C_2*integral(t=2..x,dt/log(t)^2)
PROG
(PARI) Li_2(x) = intnum(t=2, x, 2*0.660161815846869573927812110014555778432623/log(t)^2)
CROSSREFS
KEYWORD
nonn
AUTHOR
Cino Hilliard, Nov 22 2008
STATUS
approved