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 A115747 #16 Jul 14 2015 08:29:46
%S A115747 18,20,88,368,1504,24448,98048,5238976,25161728,2730992944,
%T A115747 33995232256,412316336128,1391737114624,7732492570624
%N A115747 Numbers n such that phi(n) + sigma(n) = 5/2*n.
%C A115747 If p = 3*2^(m-1)-1 is an odd prime then 2^m*p is in the sequence because phi(2^m*p) = 2^(m-1)*(3*2^(m-1)-2), sigma(2^m*p) = (2^(m+1)-1)*(3*2^(m-1)) so phi(2^m*p)+sigma(2^m*p) = 2^(m-1)*(3* 2^(m-1)-2)+(2^(m+1)-1)*(3*2^(m-1)) = 3*2^(2m-2)-2^m+3*2^(2m)-3*2^ (m-1) = 2^(m-1)*(3*2^(m-1)-2+3*2^(m+1)-3) = 2^(m-1)*(3*5*2^(m-1)-5) = 5/2*2^m*(3*2^(m-1)-1) = 5/2*(2^m*p). Except 18 & 5238976 all known terms of the sequence are of the form 2^m*(3*2^(m-1)-1), where (3*2^(m-1)-1) is prime.
%C A115747 a(15) > 10^13. - _Giovanni Resta_, Jul 13 2015
%e A115747 25161728 is in the sequence because phi(25161728) + sigma(25161728) = 12578816 + 50325504 = 5/2*25161728.
%t A115747 Do[If[DivisorSigma[1,n]+EulerPhi[n]==5/2*n,Print[n]],{n,200000000}]
%o A115747 (PARI) isok(n) = eulerphi(n) + sigma(n) == 5*n/2; \\ _Michel Marcus_, Jul 14 2015
%Y A115747 Cf. A002235.
%K A115747 more,nonn
%O A115747 1,1
%A A115747 _Farideh Firoozbakht_, Feb 12 2006
%E A115747 a(10)-a(12) from _Donovan Johnson_, Feb 29 2012
%E A115747 a(13) from _Donovan Johnson_, Apr 04 2012
%E A115747 a(14) from _Giovanni Resta_, Jul 13 2015