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 A136539 #6 Nov 03 2012 17:21:41
%S A136539 76,1264,327424,5241856,83881984,1342160896,343597121536
%N A136539 Numbers n such that n=6*phi(n)-sigma(n).
%C A136539 If 5*2^n-1 is prime (that is, n is in A001770) then m = 2^n*(5*2^n-1) is in the sequence. Proof: 6*phi(m)-sigma(m) = 6*2^(n-1)*(5*2^n-2) -(2^(n+1)-1)*5*2^n = 30*2^(2n-1)-6*2^n-5*2^(2n+1)+5*2^n = 5*2^(2n)-2^n = 2^n(5*2^n-1) = m.
%C A136539 The first seven terms of the sequence are of such form, with n=2, 4, 8, 10, 12, 14, 18. Are all terms of the sequence of this form?
%C A136539 a(8) > 10^12. - _Giovanni Resta_, Nov 03 2012
%F A136539 a(n) = 2^k*(5*2^k-1) = A084213(k+1) with k = A001770(n), for n = 1,...,7. - _M. F. Hasler_, Nov 03 2012
%e A136539 6*phi(76)-sigma(76)=6*36-140=76 so 76 is in the sequence.
%t A136539 Do[If[n==6*EulerPhi[n]-DivisorSigma[1,n],Print[n]],{n,85000000}]
%Y A136539 Cf. A001770, A136540.
%K A136539 more,nonn
%O A136539 1,1
%A A136539 _Farideh Firoozbakht_, Jan 05 2008, Feb 01 2008
%E A136539 a(7) from _Giovanni Resta_, Nov 03 2012