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