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 A230774 #21 Nov 04 2024 17:30:03
%S A230774 1,1,1,1,2,2,2,2,2,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,
%T A230774 4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
%U A230774 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5
%N A230774 Number of primes less than first prime above square root of n.
%C A230774 Or repeat k (prime(k)^2 - prime(k-1)^2) times, with prime(0) set to 0 for k = 1.
%C A230774 This sequence is useful to compute A055399 for prime numbers.
%H A230774 Jean-Christophe Hervé, <a href="/A230774/b230774.txt">Table of n, a(n) for n = 1..10000</a>
%F A230774 Repeat 1 prime(1)^2 = 4 times; for k>1, repeat k (prime(k)^2-prime(k-1)^2) = A050216(k-1) times.
%F A230774 a(n) - A056811(n) = characteristic function of squares of primes.
%e A230774 a(5) = a(6) = a(7) = a(8) = a(9) = 2 because prime(1) = 2 < sqrt(5 to 9) <= prime(2) = 3.
%t A230774 Table[1 + PrimePi[Sqrt[n-1]], {n, 100}] (* _Alonso del Arte_, Nov 01 2013 *)
%o A230774 (Python)
%o A230774 from math import isqrt
%o A230774 from sympy import primepi
%o A230774 def A230774(n): return primepi(isqrt(n-1))+1 # _Chai Wah Wu_, Nov 04 2024
%Y A230774 Cf. A050216, A056811, A055399, A230775.
%K A230774 nonn,easy
%O A230774 1,5
%A A230774 _Jean-Christophe Hervé_, Nov 01 2013