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A045529 a(n+1) = 5*a(n)^3 - 3*a(n), a(0) = 1.

%I A045529 #67 Jan 05 2025 19:51:35
%S A045529 1,2,34,196418,37889062373143906,
%T A045529 271964099255182923543922814194423915162591622175362
%N A045529 a(n+1) = 5*a(n)^3 - 3*a(n), a(0) = 1.
%C A045529 The next term, a(6), has 153 digits. - _Harvey P. Dale_, Oct 24 2011
%H A045529 Seiichi Manyama, <a href="/A045529/b045529.txt">Table of n, a(n) for n = 0..7</a>
%H A045529 Daniel Duverney and Takeshi Kurosawa, <a href="https://doi.org/10.1007/s40993-022-00366-1">Transcendence of infinite products involving Fibonacci and Lucas numbers</a>, Research in Number Theory, Vol. 8 (2002), Article 68.
%H A045529 Zalman Usiskin, <a href="https://fq.math.ca/Scanned/11-3/elementary11-3.pdf">Problem B-265</a>, Elementary Problems and Solutions, The Fibonacci Quarterly, Vol. 11, No. 3 (1973), p. 333; <a href="https://web.archive.org/web/2024*/https://www.fq.math.ca/Scanned/12-3/elementary12-3.pdf">Fibonacci Numbers for Powers of 3</a>, Solution to Problem B-265 by Ralph Garfield and David Zeitlin, ibid., Vol. 12, No. 3 (1974), p. 315.
%F A045529 The first example I know in which a(n) can be expressed as (4/5)^(1/2)*cosh(3^n*arccosh((5/4)^(1/2))).
%F A045529 a(n) = Fibonacci(3^n). - _Leroy Quet_, Mar 17 2002
%F A045529 a(n+1) = a(n)*A002814(n+1). - _Lekraj Beedassy_, Jun 16 2003
%F A045529 a(n) = (phi^(3^n) - (1 - phi)^(3^n))/sqrt(5), where phi is the golden ratio (A001622). - _Artur Jasinski_, Oct 05 2008
%F A045529 a(n) = Product_{k=0..n-1} (Lucas(2*3^k) - 1) (Usiskin, 1973). - _Amiram Eldar_, Jan 29 2022
%F A045529 From _Peter Bala_, Nov 24 2022: (Start)
%F A045529 a(2*n+2) == a(2*n) (mod 3^(2*n+1)); a(2*n+3) == a(2*n+1) (mod 3^(2*n+2));
%F A045529 a(2*n+1) + a(2*n) == 0 (mod 3^(2*n+1)).
%F A045529 a(2*n) == 1 (mod 3) and a(2*n+1) == 2 (mod 3).
%F A045529 5*a(n)^2 == 2 (mod 3^(n+1)).
%F A045529 In the ring of 3-adic integers, the sequences {a(2*n)} and {a(2*n+1)} are both Cauchy sequences and converge to the pair of 3-adic roots of the quadratic equation 5*x^2 - 2 = 0. (End)
%F A045529 From _Amiram Eldar_, Jan 07 2023: (Start)
%F A045529 Product_{n>=1} (1 + 2/(sqrt(5)*a(n)-1)) = phi (A001622).
%F A045529 Product_{n>=1} (1 - 2/(sqrt(5)*a(n)+1)) = 1/phi (A094214).
%F A045529 Both formulas are from Duverney and Kurosawa (2022). (End)
%p A045529 a := proc(n) option remember; if n = 0 then 1 else 5*a(n-1)^3 - 3*a(n-1) end if; end:
%p A045529 seq(a(n), n = 0..5); # _Peter Bala_, Nov 24 2022
%t A045529 G = (1 + Sqrt[5])/2; Table[Expand[(G^(3^n) - (1 - G)^(3^n))/Sqrt[5]], {n, 1, 7}] (* _Artur Jasinski_, Oct 05 2008 *)
%t A045529 Table[Round[(4/5)^(1/2)*Cosh[3^n*ArcCosh[((5/4)^(1/2))]]], {n, 1, 4}] (* _Artur Jasinski_, Oct 05 2008 *)
%t A045529 RecurrenceTable[{a[0]==1,a[n]==5a[n-1]^3-3a[n-1]},a[n],{n,6}] (* _Harvey P. Dale_, Oct 24 2011 *)
%t A045529 NestList[5#^3-3#&,1,5] (* _Harvey P. Dale_, Dec 21 2014 *)
%o A045529 (Maxima) A045529(n):=fib(3^n)$
%o A045529 makelist(A045529(n),n,0,10); /* _Martin Ettl_, Nov 12 2012 */
%Y A045529 Cf. (k^n)-th Fibonacci number: A058635 (k=2), this sequence (k=3), A145231 (k=4), A145232 (k=5), A145233 (k=6), A145234 (k=7), A250487 (k=8), A250488 (k=9), A250489 (k=10).
%Y A045529 Cf. A000032, A000045, A001622, A001999, A002814, A006267, A094214.
%K A045529 nonn,easy
%O A045529 0,2
%A A045529 _Jose Eduardo Blazek_, Dec 11 1999