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

1,2

Comments

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

Links

Table of n, a(n) for n=1..25.

Formula

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

Example

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

Prog

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

Crossrefs

Cf. A051027, A272412, A280555.

Sequence in context: A261821 A151587 A193137 * A134368 A152840 A003832

Adjacent sequences: A280542 A280543 A280544 * A280546 A280547 A280548

Keyword

nonn

Author

Altug Alkan, Jan 05 2017

Extensions

More terms from Giovanni Resta, Jan 08 2017

Status

approved