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A124134 Positive integers n such that Fibonacci(n) = a^2 + b^2, where a, b are integers. 2
1, 2, 3, 5, 6, 7, 9, 11, 12, 13, 14, 15, 17, 19, 21, 23, 25, 26, 27, 29, 31, 33, 35, 37, 38, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 62, 63, 65, 67, 69, 71, 73, 74, 75, 77, 79, 81, 83, 85, 86, 87, 89, 91, 93, 95, 97, 98, 99, 101, 103, 105, 107, 109, 111, 113, 115, 117, 119, 121, 122, 123, 125, 127 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

All odd numbers are in this sequence, since the Fibonacci number with index 2m+1 is the sum of the squares of the two Fibonacci numbers with indices m and m+1. Those with even indices ultimately depend on certain Lucas numbers being the sum of two squares (see A124132). Joint work with Kevin O'Bryant and Dennis Eichhorn.

Numbers n such that Fibonacci(n) or Fibonacci(n)/2 is a square are only 0, 1, 2, 3, 6, 12. So a and b must be distinct and nonzero for all values of this sequence except 1, 2, 3, 6, 12. - Altug Alkan, May 04 2016

LINKS

Joerg Arndt, Table of n, a(n) for n = 1..210

FORMULA

Intersection of A000045 and A001481.

A000161(A000045(a(n))) > 0. - Reinhard Zumkeller, Oct 10 2013

EXAMPLE

14 is in the sequence because F_14=377=11^2+16^2. 16 is not in the sequence because F_16=987 is congruent to 3 mod(4) and is thus known to not be such a sum.

MATHEMATICA

Select[Range@ 128, SquaresR[2, Fibonacci@ #] > 0 &] (* Michael De Vlieger, May 04 2016 *)

PROG

(PARI) for(n=1, 10^6, t=fibonacci(n); s=sqrtint(t); forstep(i=s, 1, -1, if(issquare(t-i*i), print1(n, ", "); break))) \\ Ralf Stephan, Sep 15 2013

(PARI) is2s(n)={my(f=factor(n)); for(i=1, #f[, 1], if(f[i, 2]%2 && f[i, 1]%4==3, return(0))); 1; } \\ see A001481

for(n=1, 10^6, if(is2s(fibonacci(n)), print1(n, ", "))); \\ Joerg Arndt, Sep 15 2013

(Haskell)

a124134 n = a124134_list !! (n-1)

a124134_list = filter ((> 0) . a000161 . a000045) [1..]

-- Reinhard Zumkeller, Oct 10 2013

CROSSREFS

Cf. A124132, A000045, A000161, A001481.

Sequence in context: A018559 A057196 A080637 * A007071 A242482 A085784

Adjacent sequences:  A124131 A124132 A124133 * A124135 A124136 A124137

KEYWORD

nonn

AUTHOR

Melvin J. Knight (melknightdr(AT)verizon.net), Nov 30 2006

EXTENSIONS

More terms from Ralf Stephan, Sep 15 2013

STATUS

approved

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Last modified October 18 08:10 EDT 2021. Contains 348066 sequences. (Running on oeis4.)