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 A280545 Numbers n such that sigma(sigma(n)) is a Fibonacci number. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 19289, 7391381, 9041581, 9124081, 9589141 are prime numbers. See A280555 for other prime terms of this sequence. LINKS 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

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Last modified September 17 11:03 EDT 2019. Contains 327129 sequences. (Running on oeis4.)