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 A280555 Primes p such that sigma(sigma(p)) is a Fibonacci number. 1

%I

%S 19289,7391381,9041581,9124081,9589141,645617593711,786881099503,

%T 793374393583,188950298985689,215446003400539,228846950929339,

%U 257138974382029,265666386165589,276918720321829,280481623844131,323331286115017,326905876894417

%N Primes p such that sigma(sigma(p)) is a Fibonacci number.

%C Is this sequence infinite?

%e Prime number 7391381 is a term because sigma(sigma(7391381)) = 14930352 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(n+1))&&isprime(n);

%Y Cf. A008333, A051027, A272412, A280545.

%K nonn

%O 1,1

%A _Altug Alkan_, Jan 05 2017

%E Terms confirmed by _Giovanni Resta_, Jan 07 2017

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Last modified October 23 23:51 EDT 2019. Contains 328379 sequences. (Running on oeis4.)