OFFSET
1,1
COMMENTS
Fibonacci numbers that are the sum of 4 but no fewer nonzero squares. See first comment in A004215.
Corresponding index of Fibonacci numbers are 10, 22, 34, 46, 58, 70, 82, 84, 94, 106, 118, 130, 142, 154, 166, 178, 180, 190, 202, 214, 226, 238, 250, 262, 274, 276, 286, 298, 310, 322, ...
First differences of corresponding indices are 12, 12, 12, 12, 12, 12, 2, 10, 12, 12, 12, 12, 12, 12, 12, 2, 10, 12, 12, 12, 12, ...
From Robert Israel, Jan 17 2016: (Start)
A000045(10+12*k) == 7 (mod 8), so these are in the sequence.
A000045(84+96*k) == 4^2*7 (mod 4^2*8), so these are in the sequence.
It appears that A000045((84+96*k)*4^j) == 4^(j+2)*7 (mod 4^(j+2)*8) for j,k>=0, so these are in the sequence. (End).
LINKS
Robert Israel, Table of n, a(n) for n = 1..970
R. M. Grassl, Problem B-226, Elementary Problems and Solutions, The Fibonacci Quarterly, Vol. 10, No. 2 (1972), p. 218; Fibonacci Sum of Four Squares, Solution to Problem B-226 by Paul S. Bruckman, ibid., Vol. 11, No. 2 (1973), p. 106.
EXAMPLE
Fibonacci number 21 is not a term of this sequence because 21 = 1^2 + 2^2 + 4^2.
55 is a term because it is a Fibonacci number and there is no integer values of x, y and z for the equation 55 = x^2 + y^2 + z^2.
MAPLE
is004215:= proc(n) local t;
t:= padic:-ordp(n, 2);
if t::odd then false else evalb(n/2^t mod 8 = 7) fi
end proc:
select(is004215, [seq(combinat:-fibonacci(n), n=2..1000)]); # Robert Israel, Jan 17 2016
MATHEMATICA
Select[Fibonacci[Range[160]], EvenQ[(e = IntegerExponent[#, 2])] && Mod[#/2^e, 8] == 7 &] (* Amiram Eldar, Jan 29 2022 *)
PROG
(PARI) isA004215(n) = { my(fouri, j) ; fouri=1 ; while( n >=7*fouri, if( n % fouri ==0, j= n/fouri -7 ; if( j % 8 ==0, return(1) ) ; ) ; fouri *= 4 ; ) ; return(0) ; } { for(n=1, 400, if(isA004215(n), print1(n, ", ") ; ) ; ) ; }
f(n) = fibonacci(n);
for(n=0, 1e3, if(isA004215(f(n)), print1(f(n), ", ")));
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Jan 14 2016
STATUS
approved