This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A066339 #15 Apr 28 2021 23:20:10
%S A066339 0,0,0,0,1,1,1,1,1,1,1,1,2,2,2,2,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,
%T A066339 4,4,5,5,5,5,6,6,6,6,6,6,6,6,6,6,6,6,7,7,7,7,7,7,7,7,8,8,8,8,8,8,8,8,
%U A066339 8,8,8,8,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,10,10,10,10,10,10,10,10,11,11,11
%N A066339 Number of primes p of the form 4m+1 with p <= n.
%C A066339 Asymptotic expansion: a(n) ~ pi(n)/2 ~ n/(2log(n)) (pi(n) is in sequence A000720).
%C A066339 Partial sums of A079260. - _Reinhard Zumkeller_, Feb 06 2014
%H A066339 T. D. Noe, <a href="/A066339/b066339.txt">Table of n, a(n) for n=1..10000</a>
%H A066339 R. Breusch, <a href="http://projecteuclid.org/Dienst/UI/1.0/Summarize/euclid.mmj/1028999182">An Asymptotic Formula for Primes Of The Form 4n+1</a>
%F A066339 a(n) + A066490(n) = A000720(n) - 1 for n >= 2. - _Jianing Song_, Apr 28 2021
%t A066339 Table[ Length[ Select[ Union[ Table[ Prime[ PrimePi[i]], {i, 2, n}]], Mod[ #, 4] == 1 & ]], {n, 2, 100} ]
%o A066339 (PARI) for(n=1,200,print1(sum(i=1,n,if((i*isprime(i)-1)%4,0,1)),","))
%o A066339 (Haskell)
%o A066339 a066339 n = a066339_list !! (n-1)
%o A066339 a066339_list = scanl1 (+) $ map a079260 [1..]
%o A066339 -- _Reinhard Zumkeller_, Feb 06 2014
%Y A066339 Cf. A000720, A066490, A296021.
%K A066339 nonn
%O A066339 1,13
%A A066339 Sharon Sela (sharonsela(AT)hotmail.com), Jan 01 2002
%E A066339 More terms from _Robert G. Wilson v_, Jan 03 2002