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 A231484 #20 Dec 30 2023 21:59:48 %S A231484 1,81,1476326929,10994571025,61436066769,98551417041,119582481249, %T A231484 141447449025,220545762129,388895668225,619568914129,890560253025, %U A231484 970952066161,2580138650961,3076652813521,3739640454225,4138876942929,4758545225281,9262289646801,26494089912081 %N A231484 Numbers n such that n, sigma(n) and sigma(sigma(n)) are odd. %C A231484 Contains exactly odd terms of A008848. %C A231484 Odd n is a term if and only if both n and sigma(n) are perfect squares. %H A231484 Donovan Johnson, <a href="/A231484/b231484.txt">Table of n, a(n) for n = 1..125</a> %H A231484 Vladimir Letsko <a href="http://www-old.fizmat.vspu.ru/doku.php?id=marathon:problem_180">Mathematical Marathon, problem 180</a> (in Russian) %e A231484 81 is a term because 81, sigma(81) = 121 and sigma(sigma(81)) = 133 are all odd. %o A231484 (PARI) forstep(r=1, 5147241, 2, n=r^2; if(sigma(sigma(n))%2<>0, print1(n ", "))) \\ _Donovan Johnson_, Nov 09 2013 %o A231484 (PARI) forstep(r=1, 5147241, 2, if(!isprime(r)&&issquare(sigma(n=r^2)), print1(n", "))) \\ _Charles R Greathouse IV_, Nov 12 2013 %Y A231484 Cf. A008848. %K A231484 nonn %O A231484 1,2 %A A231484 _Vladimir Letsko_, Nov 09 2013 %E A231484 a(5) corrected by _Donovan Johnson_, Nov 09 2013