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 A249485 #23 Sep 08 2022 08:46:10 %S A249485 3,4,8,9,16,18,25,32,36,72,121,144,169,196,200,225,242,256,338,361, %T A249485 392,441,484,512,578,676,729,800,882,900,961,968,1058,1152,1250,1521, %U A249485 1681,1800,2048,2312,2592,2601,2738,2809,2916,3136,3249,3362,3481,3528,3600 %N A249485 Numbers n such that sigma(n) - 2 is prime. %C A249485 The only prime term is 3. %C A249485 a(n) is a square or twice a square for n > 1. If a(n) is odd and n > 1, then a(n) must be a square. - _Altug Alkan_, Jul 16 2016 %H A249485 Charles R Greathouse IV, <a href="/A249485/b249485.txt">Table of n, a(n) for n = 1..10000</a> %e A249485 Number 8 is in the sequence because sigma(8)-2 = 15-2 = 13 (prime). %t A249485 Select[Range[4000], PrimeQ[DivisorSigma[1, #] - 2] &] (* _Vincenzo Librandi_, Nov 14 2014 *) %o A249485 (Magma) [n: n in[1..100000] | IsPrime(SumOfDivisors(n)- 2)] %o A249485 (PARI) for(n=1,10000,if(isprime(sigma(n)-2),print1(n,", "))) \\ _Derek Orr_, Nov 13 2014 %o A249485 (PARI) list(lim)=my(v=List([3]),t); for(n=2,sqrtint(lim\1), if(isprime(sigma(t=n^2)-2), listput(v,t))); for(n=2,sqrtint(lim\2), if(isprime(sigma(t=2*n^2)-2), listput(v,t))); Set(v) \\ _Charles R Greathouse IV_, Jul 16 2016 %Y A249485 Cf. A000040, A000203. %K A249485 nonn,easy %O A249485 1,1 %A A249485 _Jaroslav Krizek_, Nov 13 2014