OFFSET
0,2
LINKS
Chris K. Caldwell, How Many Primes Are There?
Xavier Gourdon & Pascal Sebah, Counting the number of primes
Henri Lifchitz, Parity of Pi(n)
Thomas R. Nicely, Some Results of Computational Research in Prime Numbers [See local copy in A007053]
Tomás Oliveira e Silva, Tables of values of pi(x) and of pi2(x)
EXAMPLE
a(2)=4 because there are 4 primes up to 10^2 whose initial digit is 1 (11, 13, 17 and 19).
MATHEMATICA
f[n_] := f[n] = PrimePi[2*10^n] - PrimePi[10^n] + f[n - 1]; f[0] = 0; Table[ f[n], {n, 0, 13}]
CROSSREFS
KEYWORD
base,hard,nonn
AUTHOR
Shyam Sunder Gupta, Aug 14 2002
EXTENSIONS
Edited and extended by Robert G. Wilson v, Aug 29 2002
a(20)-a(21) added by David Baugh, Mar 21 2015
a(22) from Chai Wah Wu, Sep 18 2018
STATUS
approved