OFFSET
1,3
COMMENTS
Negated first differences of A007598.
Real part of (F(n-1) + i*F(n))^2. Corresponding imaginary part = A079472(n); e.g., (3 + 5i)^2 = (-16 + 30i) where 30 = A079472(5). Consider a(n) and A079472(n) as legs of a Pythagorean triangle; then hypotenuse = corresponding n-th term in the sequence (1, 2, 5, 13, ...; i.e., odd-indexed Fibonacci terms). a(n)/a(n-1) tends to Phi^2.
REFERENCES
Daniele Corradetti, La Metafisica del Numero, 2008
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (2,2,-1).
FORMULA
a(n) = Re(F(n-1) + F(n)*i)^2 = (F(n-1))^2 - (F(n))^2.
G.f.: (1-3*x)/((1+x)*(1 - 3*x + x^2)). - Paul Barry, Oct 13 2006
a(n) = (4*(-1)^n - |A098149(n)|)/5. - R. J. Mathar, Jan 13 2011
EXAMPLE
a(5) = -16 since Re(3 + 5i)^2 = (-16 + 30i).
a(5) = -16 = 3^2 - 5^2.
MAPLE
A121646 := proc(n)
combinat[fibonacci](n+1)*combinat[fibonacci](n-2) ;
-% ;
end proc:
seq(A121646(n), n=1..10) ; # R. J. Mathar, Jun 22 2017
MATHEMATICA
f[n_] := Re[(Fibonacci[n - 1] + I*Fibonacci[n])^2]; Array[f, 29] (* Robert G. Wilson v, Aug 16 2006 *)
lst={}; Do[a1=Fibonacci[n]*Fibonacci[n+1]; a2=Fibonacci[n+1]*Fibonacci[n+2]; AppendTo[lst, 3*a1-a2], {n, 0, 60}]; lst (* Vladimir Joseph Stephan Orlovsky, Nov 17 2009 *)
Table[-Fibonacci[n-2]*Fibonacci[n+1], {n, 1, 40}] (* Vladimir Joseph Stephan Orlovsky, Nov 17 2009 *)
-Differences[Fibonacci[Range[0, 30]]^2] (* Harvey P. Dale, Nov 01 2022 *)
PROG
(PARI) a(n) = fibonacci(n-1)^2 - fibonacci(n)^2 \\ Charles R Greathouse IV, Jun 11 2015
(Magma) [-Fibonacci(n-2)*Fibonacci(n+1): n in [1..40]]; // G. C. Greubel, Jan 07 2019
(Sage) [-fibonacci(n-2)*fibonacci(n+1) for n in (1..40)] # G. C. Greubel, Jan 07 2019
(GAP) List([1..40], n -> -Fibonacci(n-2)*Fibonacci(n+1)); # G. C. Greubel, Jan 07 2019
CROSSREFS
KEYWORD
sign,easy
AUTHOR
Gary W. Adamson, Aug 13 2006
STATUS
approved