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 A152991 #13 Oct 20 2021 19:49:35
%S A152991 1,2,2,5,3,9,4,11,9,14,7,23,8,18,18,25,11,32,12,34,24,28,15,51,22,33,
%T A152991 31,47,20,62,21,52,37,43,37,80,26,48,44,78,29,83,30,70,64,58,33,109,
%U A152991 42,78,57,83,38,104,56,104,64,74,43,151,44,78,86,109,66,126,49,107,77,125,52
%N A152991 a(n) = sigma(n) - pi(n).
%C A152991 a(n) = A000203(n) - A000720(n). - _Omar E. Pol_, Dec 21 2008
%H A152991 Michel Marcus, <a href="/A152991/b152991.txt">Table of n, a(n) for n = 1..5000</a>
%e A152991 a(15) = 24 - 6 = 18 because the sum of the divisors of 15 is 1 + 3 + 5 + 15 = 24 and there are 6 primes not exceeding 15 (2, 3, 5, 7, 11 and 13). - _Emeric Deutsch_, Dec 29 2008
%p A152991 with(numtheory): seq(sigma(n)-pi(n), n = 1 .. 80); # _Emeric Deutsch_, Dec 29 2008
%t A152991 Table[DivisorSigma[1,n]-PrimePi[n],{n,80}] (* _Harvey P. Dale_, Oct 20 2021 *)
%o A152991 (PARI) a(n) = sigma(n) - primepi(n); \\ _Michel Marcus_, Jun 18 2019
%Y A152991 Cf. A000203, A000720, A152770.
%K A152991 easy,nonn
%O A152991 1,2
%A A152991 _Omar E. Pol_, Dec 19 2008
%E A152991 Corrected and extended by _Emeric Deutsch_, Dec 29 2008