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 A306492 #17 Dec 09 2020 01:13:56
%S A306492 3600,17424,22500,32400,72900,291600,345744,360000,476100,518400,
%T A306492 562500,656100,685584,756900,1040400,1382976,1411344,1742400,1904400,
%U A306492 1988100,2073600,2250000,2340900,2624400,3027600,3111696,4161600,4284900,5760000,6051600,6170256,6200100,6969600
%N A306492 Numbers k such that sigma(k) - 3k is prime.
%H A306492 Amiram Eldar, <a href="/A306492/b306492.txt">Table of n, a(n) for n = 1..1000</a>
%e A306492 The divisors of 3600 are {1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 25, 30, 36, 40, 45, 48, 50, 60, 72, 75, 80, 90, 100, 120, 144, 150, 180, 200, 225, 240, 300, 360, 400, 450, 600, 720, 900, 1200, 1800, 3600}. sigma(3600) - 3 * 3600 = 12493 - 10800 = 1693, which is prime.
%p A306492 with(numtheory): b := []: for n from 3 to 1000000 do t1 := divisors(n); t2 := convert(t1, list); t3 := add(t2[i], i=1..nops(t2)); if isprime(t3-3*n) then b := [op(b), n]; fi; od: b;
%t A306492 f[n_]:=Plus@@Divisors[n]-3*n; lst={}; Do[a=f[n]; If[PrimeQ[a], AppendTo[lst, n]], {n, 9!}]; lst
%t A306492 Select[Range[1000000], DivisorSigma[1,#] > 3*# && PrimeQ[DivisorSigma[1,#] - 3*#] &] (* _Vaclav Kotesovec_, Feb 23 2019 *)
%o A306492 (PARI) isok(n) = isprime(sigma(n) - 3*n); \\ _Michel Marcus_, Feb 19 2019
%Y A306492 Cf. A000203, A037020, A064271.
%K A306492 nonn
%O A306492 1,1
%A A306492 _Jan Koornstra_, Feb 19 2019
%E A306492 More terms from _Michel Marcus_, Feb 19 2019