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A245236 Numbers n such that the Fibonacci number F(n) satisfies F(n)^2+1 = f1*f2 where f1, f2 are prime Fibonacci numbers. 4
4, 5, 6, 9, 12, 15, 45, 432, 570 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Or index i of any Fibonacci number F(i) such that F(i-1) and F(i+1) are primes if i is even or F(i-2) and F(i+2) are primes if i is odd where F(i) is the i-th Fibonacci number.

In the general case, F(i+1)*F(i-1) = F(i)^2 + 1 if i even or F(i+2)*F(i-2) = F(i)^2 + 1 if i odd (Cassini’s identity).

The corresponding Fibonacci numbers are 3, 5, 8, 34, 144, 610, 1134903170,...

If a(10) exists, it is greater than 30000. - Robert Israel, Jul 14 2014

LINKS

Table of n, a(n) for n=1..9.

EXAMPLE

4 is a term because F(4)^2+1 = F(3)*F(5)=> 3^2+1 = 2*5;

5 is a term because F(5)^2+1 = F(3)*F(7)=> 5^2+1 = 2*13;

6 is a term because F(6)^2+1 = F(5)*F(7)=> 8^2+1 = 5*13;

9 is a term because F(9)^2+1 = F(7)*F(11)=> 34^2+1 = 13*89;

12 is a term because F(12)^2+1 = F(11)*F(13)=> 144^2+1 = 89*233;

15 is a term because F(13)*F(17)=> 610^2+1 = 233* 1597.

MAPLE

with(combinat, fibonacci):with(numtheory):nn:=1000:for n from 1 to nn do:if (type(fibonacci(n+1), prime) and type(fibonacci(n-1), prime) and irem(n, 2)=0) or (type(fibonacci(n+2), prime) and type(fibonacci(n-2), prime) and irem(n, 2)=1) then print(n):else fi:od:

# Alternative:

filter:= proc(n) uses combinat;

    if n::even then isprime(n-1) and isprime(n+1) and isprime(fibonacci(n-1)) and isprime(fibonacci(n+1))

  else isprime(n-2) and isprime(n+2) and isprime(fibonacci(n-2)) and isprime(fibonacci(n+2))

fi end proc:

select(filter, [$1..10^4]); # Robert Israel, Jul 14 2014

CROSSREFS

Cf. A000045, A005478, A245306.

Sequence in context: A192360 A115984 A145025 * A209122 A073263 A039013

Adjacent sequences:  A245233 A245234 A245235 * A245237 A245238 A245239

KEYWORD

nonn,hard

AUTHOR

Michel Lagneau, Jul 14 2014

STATUS

approved

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Last modified January 23 19:45 EST 2020. Contains 331175 sequences. (Running on oeis4.)