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 A133784 #16 Nov 30 2017 15:46:49
%S A133784 4,21,24,71
%N A133784 Positive integers n such that the sum of all primes <= n divides n(n+1)/2.
%C A133784 Also, positive integers n for which the sum of all primes <= n divides the sum of all nonprimes <= n.
%C A133784 Is this sequence finite?
%C A133784 No other terms below 10^13. - _Max Alekseyev_, Nov 28 2017
%C A133784 Sequence is probably complete; integral_{x=10^10..infinity} (2 log x)/x^2 dx = 4.80517... * 10^-9. - _Charles R Greathouse IV_, Mar 21 2013
%e A133784 n=21 is in this sequence since the sum of the primes <=21 is 2+3+5+7+11+13+17+19=77 and 77 divides 21*22/2=231.
%p A133784 P:=proc(n) local i,snp,sp; snp:=1; sp:=2; for i from 3 by 1 to n do if isprime(i) then sp:=sp+i; else snp:=snp+i; fi; if trunc(snp/sp)=snp/sp then print(i); fi; od; end: P(10000000);
%t A133784 Select[Range[2,100],Divisible[(#(#+1))/2,Total[Prime[Range[ PrimePi[ #]]]]]&] (* _Harvey P. Dale_, May 31 2012 *)
%o A133784 (PARI) A=1;P=0;for(n=2,1e9,A+=n;P+=isprime(n)*n;if(A%P==0,print1(n", "))) \\ _Charles R Greathouse IV_, Mar 21 2013
%Y A133784 Cf. A007504, A000217, A051349.
%K A133784 nonn
%O A133784 1,1
%A A133784 _Paolo P. Lava_ and _Giorgio Balzarotti_, Jan 21 2008
%E A133784 Edited by _Max Alekseyev_, Feb 13 2009