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 A065449 a(n) = phi(Fibonacci(n)). 12
 0, 1, 1, 1, 2, 4, 4, 12, 12, 16, 40, 88, 48, 232, 336, 240, 552, 1596, 1152, 4032, 3200, 5040, 17424, 28656, 12672, 60000, 120640, 89856, 188160, 514228, 288000, 1343296, 1217712, 1742400, 5697720, 6814080, 4396032, 23656320, 37691136 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS For n > 4, a(n) is a multiple of 4, but a proof was elusive for a number of years. According to Koshy (2001), P. L. Montgomery "provided an elegant solution using group theory" in 1977, but Montgomery's proof is not quoted in Koshy's book. Pe wonders if there is a closed form for this sequence, like there is for the Fibonacci numbers (Binet's formula). I wonder if there is a recurrence relation. - Alonso del Arte, Oct 11 2011 a(n) must be divisible by 4 for n > 4, since otherwise F(n) must be 1, 2, 4, a prime congruent to 3 modulo 4, or twice a prime congruent to 3 modulo 4. The first two happen for n = 1, 2, and 3, the third never occurs, the fourth can only occur for n = 4 since 3|F(4k) for all positive k, and the fifth never occurs since F(n) is never congruent to 6 modulo 8. - Charlie Neder, Apr 26 2019 REFERENCES Thomas Koshy, "Fibonacci and Lucas Numbers and Applications", Wiley, New York (2001) p. 413, Theorem 34.12. LINKS Amiram Eldar, Table of n, a(n) for n = 0..1408 (terms 0..466 from Harry J. Smith, terms 467..1000 from Charles R Greathouse IV) Blair Kelly, Fibonacci and Lucas Factorizations FORMULA a(n) = A000010(A000045(n)). EXAMPLE a(9) = phi(F(9)) = phi(34) = phi(2 * 17) = 16. MAPLE with(numtheory):with(combinat):a:=n->phi(fibonacci(n)): seq(a(n), n=0..38); # Zerinvary Lajos, Oct 07 2007 MATHEMATICA Table[ EulerPhi[ Fibonacci[ n]], {n, 0, 46} ] PROG (PARI) for(n=1, 75, print1(eulerphi(fibonacci(n)), ", ")) (PARI) { for (n=0, 466, if (n, a=eulerphi(fibonacci(n)), a=0); write("b065449.txt", n, " ", a) ) } \\  Harry J. Smith, Oct 20 2009 (Sage) [euler_phi(fibonacci(n))for n in range(0, 39)] # Zerinvary Lajos, Jun 06 2009 (MAGMA)  cat [EulerPhi(Fibonacci(n)): n in [1..30]]; // G. C. Greubel, Jan 18 2018 CROSSREFS Cf. A000010, A000045, A065451. Sequence in context: A292303 A000936 A319594 * A130618 A129882 A129017 Adjacent sequences:  A065446 A065447 A065448 * A065450 A065451 A065452 KEYWORD nonn,nice AUTHOR Joseph L. Pe, Nov 18 2001 EXTENSIONS More terms from several correspondents, Nov 19 2001 STATUS approved

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Last modified May 13 16:56 EDT 2021. Contains 343862 sequences. (Running on oeis4.)