%I #14 Mar 04 2015 03:53:15
%S 1,2,7,10,25,147,218,446,551,5960,14252,63337,118472,183457,951700,
%T 3458334,6284060,2581691,80743228,259753425
%N (Number of primes == 3 mod 4 less than 10^n) - (number of primes == 1 mod 4 less than 10^n).
%D Hans Riesel, Prime Numbers and Computer Methods for Factorization, 2nd ed., Birkhauser, The distribution of primes between the two series 4n+1 and 4n+3, pages 73-77, with graphs.
%H Marc Deléglise, Pierre Dusart, and Xavier-Francois Roblot, <a href="http://dx.doi.org/10.1090/S0025-5718-04-01649-7">Counting primes in residue classes</a>, Math. Comp. 73 (2004), no. 247, 1565-1575.
%H Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_256.htm">Puzzle 256, Jack Brennen old records</a>, The Prime Puzzles & Problems Connection.
%F a(n) = A091099(n) - A091098(n) = A093153(n) + 1. [_Max Alekseyev_, May 17 2009]
%e a(1) = 1 because below 10^1 3 and 7 are 3 mod 4 and 5 is 1 mod 4 and the difference is 2-1=1.
%Y Cf. A091098, A091099. A002144, A002145, A091318, A091267.
%K nonn,more
%O 1,2
%A _Enoch Haga_, Feb 23 2004
%E a(17)-a(20) from _Marc Deleglise_, Jun 28 2007