A280545 Numbers k such that sigma(sigma(k)) is a Fibonacci number.

1, 4, 13228, 19289, 187838, 201255, 281771, 308591, 7391381, 9041581, 9124081, 9589141, 193690242024, 254218445292, 306676216538, 375274847812, 402512534123, 423082506591, 425477274579, 443127377343, 456962010177, 458339382147, 476155178309, 480858185253, 484213195281

Offset: 1

Author

Altug Alkan, Jan 05 2017

Keywords

• nonn

Comments

19289, 7391381, 9041581, 9124081, 9589141 are prime numbers. See A280555 for other prime terms of this sequence.

Examples

			19289 is a term because sigma(19289) = 19290 and sigma(19290) = 46368 is a Fibonacci number.
		

Crossrefs

Cf. A051027, A272412, A280555.

Programs

PARI

isFibonacci(n)=my(k=n^2); issquare(k+=(k+1)<<2) || (n>0 && issquare(k-8));
is(n)=isFibonacci(sigma(sigma(n)));

Formula

Numbers k such that A051027(k) is in A000045. - Wesley Ivan Hurt, Apr 05 2023

Extensions

More terms from Giovanni Resta, Jan 08 2017