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 A087485 #22 Feb 19 2017 10:36:13
%S A087485 7,15,315,1155,815634435
%N A087485 Odd numbers n such that 2n - sigma(n) = 6.
%C A087485 This is a subsequence of A077374.
%C A087485 Except for the first term, all known terms of this sequence are divisible by 15. Is there a number n > 1 such that gcd(a(n),3)=1 or gcd(a(n),5)=1?
%C A087485 a(6) > 10^13. - _Giovanni Resta_, Mar 29 2013
%C A087485 Also, a subsequence of A141548. - _M. F. Hasler_, Apr 12 2015
%C A087485 The terms a(3) through a(5) are of the form a(k)*p*q, but I have proved that there is no other term of this form with k <= 5. - _M. F. Hasler_, Apr 13 2015
%C A087485 The terms are also of the form a(n) = 2*p(n) + 1, with primes p(n) = 3, 7, 157, 577, 407817217. All but the last one are such that 2*p(n) - 1 = a(n) - 2 is again prime. - _M. F. Hasler_, Nov 27 2016
%C A087485 Terms a(2..5) satisfy 2*a(n) - nextprime(sigma(a(n))) = (-1)^n, see also A067795. - _M. F. Hasler_, Feb 14 2017
%F A087485 a(3) = a(2)*3*7; a(4) = a(2)*7*11 with 7 = precprime(a(2)*2/3), 11=nextprime(a(2)*2/3); a(5) = a(4)*547*1291. - _M. F. Hasler_, Apr 13 2015
%e A087485 15 is in the sequence because 2*15-sigma(15)=6.
%t A087485 Do[If[OddQ[n]&&2n-DivisorSigma[1, n]==6, Print[n]], {n, 2*10^9}]
%o A087485 (PARI) is(n)=bittest(n,0)&&sigma(n)+6==2*n \\ _M. F. Hasler_, Apr 12 2015
%Y A087485 Cf. A077374, A087167, A088011, A088012.
%K A087485 hard,more,nonn
%O A087485 1,1
%A A087485 _Farideh Firoozbakht_, Oct 23 2003