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 A280545 #20 Apr 05 2023 08:21:37 %S A280545 1,4,13228,19289,187838,201255,281771,308591,7391381,9041581,9124081, %T A280545 9589141,193690242024,254218445292,306676216538,375274847812, %U A280545 402512534123,423082506591,425477274579,443127377343,456962010177,458339382147,476155178309,480858185253,484213195281 %N A280545 Numbers k such that sigma(sigma(k)) is a Fibonacci number. %C A280545 19289, 7391381, 9041581, 9124081, 9589141 are prime numbers. See A280555 for other prime terms of this sequence. %F A280545 Numbers k such that A051027(k) is in A000045. - _Wesley Ivan Hurt_, Apr 05 2023 %e A280545 19289 is a term because sigma(19289) = 19290 and sigma(19290) = 46368 is a Fibonacci number. %o A280545 (PARI) isFibonacci(n)=my(k=n^2); issquare(k+=(k+1)<<2) || (n>0 && issquare(k-8)); %o A280545 is(n)=isFibonacci(sigma(sigma(n))); %Y A280545 Cf. A051027, A272412, A280555. %K A280545 nonn %O A280545 1,2 %A A280545 _Altug Alkan_, Jan 05 2017 %E A280545 More terms from _Giovanni Resta_, Jan 08 2017