A064847 -id:A064847 - cp's OEIS frontend

A003686 Number of genealogical 1-2 rooted trees of height n.

1, 2, 3, 5, 11, 41, 371, 13901, 5033531, 69782910161, 351229174914190691, 24509789089655802510792656021, 8608552999157278575508415639286249242844899051

Offset: 1

Vsevolod F. Lev, c. 1998

Let u(n), v(n) be defined by u(1) = v(1) = 1, u(n+1) = u(n) + v(n) and v(n+1) = u(n)*v(n) for n >= 1; then a(n) = u(n) and A064847(n) = v(n). - Benoit Cloitre, Apr 01 2002 [Edited by Petros Hadjicostas, May 11 2020]
Consider the mapping f(a/b) = (a + b)/(a*b). Taking a = 1 and b = 1 to start with and carrying out this mapping repeatedly on each new (reduced) rational number gives the following sequence 1/1, 2/1, 3/2, 5/6, 11/30, ... The current sequence contains the numerators. - Amarnath Murthy, Mar 24 2003
An infinite coprime sequence defined by recursion. - Michael Somos, Mar 19 2004

D. Parisse, The Tower of Hanoi and the Stern-Brocot Array, Thesis, Munich, 1997.

Franklin T. Adams-Watters, Table of n, a(n) for n = 1..19
Index entries for sequences related to Stern's sequences
Index entries for sequences related to rooted trees

Cf. A001622, A001685, A064526, A064847, A070231, A070233, A070234, A094303.

I:=[1,2]; [n le 2 select I[n] else Self(n-1)+Self(n-2)*(Self(n-1)-Self(n-2)): n in [1..14]]; // Vincenzo Librandi, Jul 19 2016

RecurrenceTable[{a[1]==1, a[2]==2, a[n]==a[n-1]+a[n-2](a[n-1]-a[n-2])}, a[n],{n,15}] (* Harvey P. Dale, Jul 27 2011 *)
Re[NestList[Re@#+(1+I Re@#)Im@#&, 1+I, 15]] (* Vladimir Reshetnikov, Jul 18 2016 *)

a(n) = local(an); if(n<1, 0, an=vector(max(2,n)); an[1]=1; an[2]=2; for(k=3, n, an[k]=an[k-1] - an[k-2]^2 + an[k-1]*an[k-2]); an[n])

Limit_{n -> infinity} a(n)^phi/A064847(n) = 1, where phi = (1 + sqrt(5))/2 is the golden ratio. - Benoit Cloitre, May 08 2002
Numerator of b(n), where b(n) = 1/numerator(b(n-1)) + 1/denominator(b(n-1)) for n >= 2 with b(1) = 1.
a(n+1) = a(n) + a(1)*a(2)*...*a(n-1) for n >= 2. Also a(n+1) = a(n) + a(n-1)*(a(n) - a(n-1)) for n >= 2. In both cases, we start with a(1) = 1 and a(2) = 2.
a(n) ~ c^(phi^n), where c = 1.22508584062304325811405322247537613534139348463831009881946422737141574647... and phi = A001622 = (1+sqrt(5))/2 is the golden ratio. - Vaclav Kotesovec, May 21 2015

Additional description from Andreas M. Hinz and Daniele Parisse

A064526 Define a pair of sequences by p(0) = 0, q(0) = p(1) = q(1) = 1, q(n+1) = p(n)*q(n-1), p(n+1) = q(n+1) + q(n) for n > 0; then a(n) = p(n) and A064183(n) = q(n).

Original entry on oeis.org

0, 1, 2, 3, 5, 13, 49, 529, 21121, 10369921, 213952189441, 2214253468601687041, 473721461635593679669210030081, 1048939288228833101089604217183056027094304481281

Offset: 0

Michael Somos, Oct 07 2001

Every nonzero term is relatively prime to all others (which proves that there are infinitely many primes). See A236394 for the primes that appear.

R. Mestrovic, Euclid's theorem on the infinitude of primes: a historical survey of its proofs (300 BC--2012) and another new proof, arXiv preprint arXiv:1202.3670 [math.HO], 2012. - _N. J. A. Sloane_, Jun 13 2012
Michael Somos and R. Haas, A linked pair of sequences implies the primes are infinite, Amer. Math. Monthly, 110(6) (2003), 539-540.

Cf. A001685, A003686, A064183, A064847, A070231, A070233, A070234, A094303.

See A236394 for the primes that are produced.

Flatten[{0,1, RecurrenceTable[{a[n]==(a[n-1]^2 + a[n-2]^2 - a[n-1]*a[n-2] * (1+a[n-2]))/(1-a[n-2]), a[2]==2, a[3]==3},a,{n,2,15}]}] (* Vaclav Kotesovec, May 21 2015 *)

{a(n) = local(v); if( n<3, max(0, n), v = [1,1]; for( k=3, n, v = [v[2], v[1] * (v[1] + v[2])]); v[1] + v[2])}

{a(n) = if( n<4, max(0, n), (a(n-1)^2 + a(n-2)^2 - a(n-1) * a(n-2) * (1 + a(n-2))) / (1 - a(n-2)))}

a(n) = (a(n-1)^2 + a(n-2)^2 - a(n-1) * a(n-2) * (1 + a(n-2))) / (1 - a(n-2)) for n >= 2.

a(n) ~ c^(phi^n), where c = 1.2364241784241086061606568429916822975882631646194967549068405592472125928485... and phi = A001622 = (1+sqrt(5))/2 is the golden ratio. - Vaclav Kotesovec, May 21 2015

A070231 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)v(k) + v(k)w(k) + w(k)u(k), and w(k+1) = u(k)v(k)*w(k) for k >= 1; then a(n) = u(n).

Original entry on oeis.org

1, 3, 7, 31, 1279, 9202687, 3692849258577919, 98367959484921734629696721986125823, 3882894052327310957045599009145809243674851356642054390303168725061781159935999

Offset: 1

Benoit Cloitre, May 08 2002

Petros Hadjicostas, Table of n, a(n) for n = 1..12

Cf. A003686, A064183, A064526, A064847, A070233 (= w), A070234 (= v), A094303, A160389, A255249.

a[1] = 1; v[1] = 1; w[1] = 1; a[k_] := a[k] = a[k - 1] + v[k - 1] + w[k - 1]; v[k_] := v[k] = a[k - 1]*v[k - 1] + v[k - 1]*w[k - 1] + w[k - 1]*a[k - 1]; w[k_] := w[k] = a[k - 1]*v[k - 1]*w[k - 1]; Table[a[n], {n, 1, 9}] (* Vaclav Kotesovec, May 11 2020 *)

lista(nn) = {my(u = vector(nn)); my(v = vector(nn)); my(w = vector(nn)); u[1] = 1; v[1] = 1; w[1] = 1; for (n=2, nn, u[n] = u[n-1] + v[n-1] + w[n-1]; v[n] = u[n-1]*v[n-1] + v[n-1]*w[n-1] + w[n-1]*u[n-1]; w[n] = u[n-1]*v[n-1]*w[n-1];); u; } \\ Petros Hadjicostas, May 11 2020

Let C be the positive root of x^3 + x^2 - 2*x - 1 = 0; that is, C = 1.246979603717... = A255249. Then Lim_{n -> infinity} u(n)^(C+1)/w(n)= Lim_{n -> infinity} v(n)^C/w(n) = Lim_{n -> infinity} u(n)^B/v(n) = 1 with B = C + 1 - 1/(1 + C) = 1.8019377... = A160389. [corrected by Vaclav Kotesovec, May 11 2020]

a(n) ~ gu^((1 + C)^n), where C is defined above and gu = 1.131945853718244297... The relation between constants gu, gv (see A070234) and gw (see A070233) is gu^(1 + C) = gv^C = gw. - Vaclav Kotesovec, May 11 2020

A070233 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)v(k) + v(k)w(k) + w(k)u(k), and w(k+1) = u(k)v(k)*w(k) for k >= 1; then a(n) = w(n).

Original entry on oeis.org

1, 1, 9, 945, 8876385, 3689952451492545, 98367948795841301790914258556831105, 3882894052327309905582682317031276840071039865528864289025562807872336355445505

Offset: 1

Benoit Cloitre, May 08 2002

Next term is too large to include.

Petros Hadjicostas, Table of n, a(n) for n = 1..11

Cf. A003686, A064183, A064526, A064847, A070231 (= u), A070234 (= v), A094303, A160389, A255249.

u[1] = 1; v[1] = 1; a[1] = 1; u[k_] := u[k] = u[k - 1] + v[k - 1] + a[k - 1]; v[k_] := v[k] = u[k - 1]*v[k - 1] + v[k - 1]*a[k - 1] + a[k - 1]*u[k - 1]; a[k_] := a[k] = u[k - 1]*v[k - 1]*a[k - 1]; Table[a[n], {n, 1, 9}] (* Vaclav Kotesovec, May 11 2020 *)

lista(nn) = {my(u = vector(nn)); my(v = vector(nn)); my(w = vector(nn)); u[1] = 1; v[1] = 1; w[1] = 1; for (n=2, nn, u[n] = u[n-1] + v[n-1] + w[n-1]; v[n] = u[n-1]*v[n-1] + v[n-1]*w[n-1] + w[n-1]*u[n-1]; w[n] = u[n-1]*v[n-1]*w[n-1]; ); w; } \\ Petros Hadjicostas, May 11 2020

Let C be the positive root of x^3 + x^2 - 2*x - 1 = 0: that is, C = 1.246979603717... = A255249. Then Lim_{n -> infinity} u(n)^(C+1)/w(n)= Lim_{n -> infinity} v(n)^C/w(n) = Lim_{n -> infinity} u(n)^B/v(n) = 1 with B = C + 1 - 1/(1 + C) = 1.8019377... = A160389. [corrected by Vaclav Kotesovec, May 11 2020]

a(n) ~ gw^((C + 1)^n), where C is defined above and gw = 1.321128752475732548... The relation between constants gu (see A070231), gv (see A070234) and gw is gu^(1 + C) = gv^C = gw. - Vaclav Kotesovec, May 11 2020

A070234 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)v(k) + v(k)w(k) + w(k)u(k), and w(k+1) = u(k)v(k)*w(k); then a(n) = v(n).

Original entry on oeis.org

1, 3, 15, 303, 325023, 2896797882687, 10689080432835089614170716799, 1051462916692114532403603811392745230616355871287492722818364671

Offset: 1

Benoit Cloitre, May 08 2002

Petros Hadjicostas, Table of n, a(n) for n = 1..11

Cf. A003686, A064183, A064526, A064847, A070231 (= u), A070233 (= w), A094303, A160389, A255249.

u[1] = 1; a[1] = 1; w[1] = 1; u[k_] := u[k] = u[k - 1] + a[k - 1] + w[k - 1]; a[k_] := a[k] = u[k - 1]*a[k - 1] + a[k - 1]*w[k - 1] + w[k - 1]*u[k - 1]; w[k_] := w[k] = u[k - 1]*a[k - 1]*w[k - 1]; Table[a[n], {n, 1, 9}] (* Vaclav Kotesovec, May 11 2020 *)

lista(nn) = {my(u = vector(nn)); my(v = vector(nn)); my(w = vector(nn)); u[1] = 1; v[1] = 1; w[1] = 1; for (n=2, nn, u[n] = u[n-1] + v[n-1] + w[n-1]; v[n] = u[n-1]*v[n-1] + v[n-1]*w[n-1] + w[n-1]*u[n-1]; w[n] = u[n-1]*v[n-1]*w[n-1]; ); v; } \\ _Petros Hadjicostas, May 11 2020

Let C be the positive root of x^3 + x^2 - 2*x - 1 = 0; that is, C = 1.246979603717... = A255249. Then Lim_{n -> infinity} u(n)^(C+1)/w(n) = Lim_{n -> infinity} v(n)^C/w(n) = Lim_{n -> infinity} u(n)^B/v(n) = 1 with B = C + 1 - 1/(1 + C) = 1.8019377... = A160389. [corrected by Vaclav Kotesovec, May 11 2020]

a(n) ~ gv^((C + 1)^n), where C is defined above and gv = 1.250231610564761084... The relation between constants gu (see A070231), gv and gw (see A070233) is gu^(1 + C) = gv^C = gw. - Vaclav Kotesovec, May 11 2020

A094303 a(1) = 1, a(2) = 2, and a(n+1) = a(n) * sum of all previous terms up to a(n-1) for n >= 2.

Original entry on oeis.org

1, 2, 2, 6, 30, 330, 13530, 5019630, 69777876630, 351229105131280530, 24509789089304573335878465330, 8608552999157278550998626549630446732052243030

Offset: 1

Amarnath Murthy, Apr 29 2004

nonn

From Petros Hadjicostas, May 11 2020: (Start)
R. J. Mathar's conjecture is correct and this is identical to A064847 starting at n = 3. To see why this is the case, consider the sequences u(n) and v(n) defined by u(1) = v(1) = 1, and u(k+1) = u(k) + v(k), v(k+1) = u(k)*v(k) for k >= 1. Then u(n) = A003686(n) and v(n) = A064847(n) for n >= 1.
Then v(n) = u(n+1) - u(n), and thus Sum_{k=1..n-1} v(k) = u(n) - u(1) = u(n) - 1 for n >= 2. Then v(n-1) + ... + v(3) + (v(2) + 1) + v(1) = u(n) for n >= 3, and hence v(n)*(v(n-1) + ... + v(3) + (v(2) + 1) + v(1)) = u(n)*v(n) = v(n+1).
Since v(1) = 1 = a(1) and v(2) + 1 = 2 = a(2), the sequence (v(1), v(2) + 1, v(3), ..., v(n), ...) is identical to the current sequence. Hence, a(n) = v(n) = u(n+1) - u(n) = A003686(n+1) - A003686(n) for n >= 3. (End)

Petros Hadjicostas, Table of n, a(n) for n = 1..17

See A064847 for another version.

Cf. A003686, A064183, A064526, A070231, A070233, A070234.

nxt[{t1_,t2_,a_}]:=Module[{c=t1*a},{t1+t2,c,c}]; Join[{1},NestList[nxt,{1,2,2},10][[All,2]]] (* Harvey P. Dale, Aug 30 2020 *)

lista(nn) = { my(va = vector(nn)); va[1] = 1; va[2] = 2; for(n=3, nn, va[n] = va[n-1]*sum(k=1, n-2, va[k]);); va; } \\ Petros Hadjicostas, May 11 2020

Conjecture: a(n) = A003686(n+1) - A003686(n) for n >= 3. - R. J. Mathar, Apr 24 2007

More terms from Gareth McCaughan, Jun 10 2004

A064183 Define a pair of sequences by p(0) = 0, q(0) = p(1) = q(1) = 1, q(n+1) = p(n)*q(n-1), p(n+1) = q(n+1) + q(n) for n > 0; then a(n) = q(n) and A064526(n) = p(n).

Original entry on oeis.org

1, 1, 1, 2, 3, 10, 39, 490, 20631, 10349290, 213941840151, 2214253254659846890, 473721461633379426414550183191, 1048939288228833100615882755549676600679754298090

Offset: 0

Michael Somos, Sep 20 2001

Michael Somos and R. Haas, A linked pair of sequences implies the primes are infinite, Amer. Math. Monthly, 110(6) (2003), 539-540.

Cf. A001622, A003686, A064526, A064847, A070231, A070233, A070234, A094303, A236394.

Flatten[{1, RecurrenceTable[{a[n]==(a[n-1]+a[n-2])*a[n-2], a[1]==1, a[2]==1},a,{n,1,10}]}] (* Vaclav Kotesovec, May 21 2015 *)

{a(n) = local(v); if( n<3, n>=0, v = [1,1]; for( k=3, n, v = [v[2], v[1] * (v[1] + v[2])]); v[2])}

{a(n) = if( n<3, n>=0, (a(n-1) + a(n-2)) * a(n-2))}

a(n) = (a(n-1) + a(n-2))*a(n-2) for n >= 2.
Lim_{n -> infinity} a(n)/a(n-1)^phi = 1, where phi = A001622. - Gerald McGarvey, Aug 29 2004
a(n) ~ c^(phi^n), where c = 1.23642417842410860616065684299168229758826316461949675490684055924721259... and phi = A001622 = (1 + sqrt(5))/2 is the golden ratio. - Vaclav Kotesovec, May 21 2015

A001697 a(n+1) = a(n)(a(0) + ... + a(n)).

Original entry on oeis.org

1, 1, 2, 8, 96, 10368, 108615168, 11798392572168192, 139202068568601556987554268864512, 19377215893777651167043206536157390321290709180447278572301746176

Offset: 0

N. J. A. Sloane

Number of binary trees of height n where for each node the left subtree is at least as high as the right subtree. - Franklin T. Adams-Watters, Feb 08 2007
The next term (a(10)) has 129 digits. - Harvey P. Dale, Jan 24 2016
Number of plane trees where the root has exactly n children and the i-th child of any node has at most i-1 children. - David Eppstein, Dec 18 2021

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

John Cerkan, Table of n, a(n) for n = 0..12
A. V. Aho and N. J. A. Sloane, Some doubly exponential sequences, Fibonacci Quarterly, Vol. 11, No. 4 (1973), pp. 429-437.
A. V. Aho and N. J. A. Sloane, Some doubly exponential sequences, Fibonacci Quarterly, Vol. 11, No. 4 (1973), pp. 429-437 (original plus references that F.Q. forgot to include - see last page!)
Daniel Duverney, Takeshi Kurosawa, Iekata Shiokawa, Transformation formulas of finite sums into continued fractions, arXiv:1912.12565 [math.NT], 2019.
Index entries for sequences of form a(n+1)=a(n)^2 + ...
Index to divisibility sequences

a(n) = A039941(2*n+1); first differences of A001696 give this sequence.
Cf. A002658, A001699.
Cf. A064847.

a001697 n = a001697_list !! n
a001697_list = 1 : 1 : f [1,1] where
   f xs@(x:_) = y : f (y : xs) where y = x * sum xs
-- Reinhard Zumkeller, Apr 29 2013

[n le 2 select 1 else Self(n-1)^2*(1+1/Self(n-2)): n in [1..12]]; // Vincenzo Librandi, Nov 25 2015

a[0] = 1; a[1] = 1; a[n_] := a[n] = a[n - 1]^2*(1 + 1/a[n - 2]); Table[a[n], {n, 0, 9}]  (* Jean-François Alcover, Jul 02 2013 *)
nxt[{t_,a_}]:={t+t*a,t*a}; Transpose[NestList[nxt,{1,1},10]][[2]] (* Harvey P. Dale, Jan 24 2016 *)

a(n)=if(n<2,n >= 0,a(n-1)^2*(1+1/a(n-2)))

a(n) ~ c^(2^n), where c = 1.3352454783981919948826893254756974184778316104856161827213437094446034867599... . - Vaclav Kotesovec, May 21 2015

Additional comments from Michael Somos, May 19 2000

A296516 a(n) is the number of terms in expanded form of bivariate polynomial Q_n, where (P_n, Q_n) is defined by: P_0 = x, Q_0 = y, P_(n+1) = P_n + Q_n, Q_(n+1) = P_n * Q_n.

Original entry on oeis.org

1, 1, 2, 5, 14, 40, 111, 300, 797, 2098, 5499, 14389, 37634, 98435, 257516, 673827, 1763460, 4615686, 12082137, 31628294

Offset: 0

Luc Rousseau, Feb 27 2018

Programs based on the direct application of the definition quickly reach a limitation by combinatorial explosion, hence this short list of values in section Data. The first conjectured formula (see Formulas) obtained by the observation of a pattern in the 2D shape of Q_n (as drawn in the Examples) is more computationally efficient and makes it possible to produce a significantly longer list of (nonguaranteed) values: see attached a-file in b-file format, section Links.

A003686 and A064847 are values of P_n and Q_n at x=y=1 (i.e., sums of coefficients in these polynomials). At x=2, y=1 (or vice versa) P_n and Q_n seem to give same sequences but shifted. At x=y=-1, P_n seems to give A000058 negated interleaved with -1's, while Q_n seems to give A007018 interleaved with the same sequence negated. - Andrey Zabolotskiy, May 22 2018

			Q_0 = y -> one term -> a(0) = 1;
Q_1 = x*y -> one term -> a(1) = 1;
Q_2 = x^2*y + x*y^2 -> two terms -> a(2) = 2;
Q_3 = x^3*y + 2*x^2*y^2 + x^3*y^2 + x*y^3 + x^2*y^3 -> five terms -> a(3) = 5;
...
Locations of terms in 2D arrays indexed by the exponents of x and y:
   0: .   1: ..   2: ...   3: ....   4: ......   5: .........
      X      .X      ..X      ...X      ....X.      .....X...
                     .X.      ..XX      ...XXX      ....XXXX.
                              .XX.      ..XXXX      ...XXXXXX
                                        .XXXX.      ..XXXXXXX
                                        ..XX..      .XXXXXXXX
                                                    ..XXXXXX.
                                                    ..XXXXX..
                                                    ...XXX...

Luc Rousseau, 201 terms obtained with conjectured formula.
Rémy Sigrist, Colored scatterplot of the points (i, j) such that the coefficient of x^i*y^j in Q_14 is nonzero (where the color is function of the coefficient of x^i*y^j in Q_14).

Cf. A000045, A000217, A001622, A033192, A003686, A064847, A000058, A007018.

Cf. A005207 (which with +1 added appears to be to P_n as a(n) is to Q_n).

{p[0], q[0]} := {x, y};
{p[n_], q[n_]} := {p[n - 1] + q[n - 1], p[n - 1] q[n - 1]};
a[n_] := Length@CoefficientRules[q[n]];
Table[a[n], {n, 0, 10}] (* Andrey Zabolotskiy, Peter Luschny, May 30 2018 *)
From Luc Rousseau, Feb 27 2018: (Start)
(* conjectured: *)
T[n_] := n*(n + 1)/2
F[n_] := Fibonacci[n]
V[n_] := Sum[F[k]*(Sum[(n - 2 - l)*F[l], {l, k + 1, n - 3}]), {k, 1, n - 4}]
W[n_] := Sum[(n - 2 - l)*T[F[l]], {l, 1, n - 3}]
AA[n_] := (F[n + 1])^2 - T[n - 1] - T[F[n - 1]] - 2*V[n] - 2 W[n]
Table[AA[n], {n, 1, 50}]
(End)

def A296516(n):
    P, Q = {(1,0)}, {(0,1)}
    for _ in range(n): P, Q = P|Q, set((p[0]+q[0],p[1]+q[1]) for p in P for q in Q)
    return len(Q) # Chai Wah Wu, Oct 18 2021

Conjectured:
a(n) = F(n+1)^2 - T(n-1) - T(F(n-1)) - 2*V(n) - 2*W(n) for n > 0, where
  F(n) is the n-th Fibonacci number,
  T(n) is the n-th triangular number,
  V(n) = Sum_{i=1..n-4} F(i)*(Sum_{j=i+1..n-3} (n-2-j)*F(j)),
  W(n) = Sum_{i=1..n-3} (n-2-i)*T(F(i)).
Or:
a(n) = F(n+1)^2 - T(n-1) - T(F(n-1)) - [n>=4]*U(n) where [] is the Iverson bracket, and U(n) = F(n-1)*F(n-3) + F(n+1)*(2*F(n-4)-1) + 5 - 8*(n mod 2).
Conjectures from Colin Barker, Mar 28 2018: (Start)
G.f.: 1 + x*(1 - 5*x + 9*x^2 - 5*x^3 - 2*x^4 + x^5) / ((1 - x)^3*(1 - 3*x + x^2)*(1 - x - x^2)).
a(n) = 7*a(n-1) - 18*a(n-2) + 20*a(n-3) - 6*a(n-4) - 6*a(n-5) + 5*a(n-6) - a(n-7) for n > 7.
(End)
From Luc Rousseau, Mar 30 2018: (Start)
Derived from the conjectured order-7 linear recurrence above, for n > 0,
a(n) = -(1/2)*n^2 + (1/2)*n - 1 + ((2+phi)/10)*(phi^2)^n + ((4-3*phi)/10)*(-phi^(-1))^n + ((1+3*phi)/10)*phi^n + ((3-phi)/10)*(phi^(-2))^n,
where phi denotes the golden ratio and lim_{n->oo} a(n+1)/a(n) = phi^2.
a(n) = (F(2*n+1) + F(n+2) - n^2 + n - 2) / 2.
(End)

a(15)-a(19) from Andrey Zabolotskiy, May 30 2018

cp's OEIS Frontend

A003686 Number of genealogical 1-2 rooted trees of height n.

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A064526 Define a pair of sequences by p(0) = 0, q(0) = p(1) = q(1) = 1, q(n+1) = p(n)*q(n-1), p(n+1) = q(n+1) + q(n) for n > 0; then a(n) = p(n) and A064183(n) = q(n).

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A070231 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)*v(k) + v(k)*w(k) + w(k)*u(k), and w(k+1) = u(k)*v(k)*w(k) for k >= 1; then a(n) = u(n).

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A070233 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)*v(k) + v(k)*w(k) + w(k)*u(k), and w(k+1) = u(k)*v(k)*w(k) for k >= 1; then a(n) = w(n).

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A070234 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)*v(k) + v(k)*w(k) + w(k)*u(k), and w(k+1) = u(k)*v(k)*w(k); then a(n) = v(n).

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A094303 a(1) = 1, a(2) = 2, and a(n+1) = a(n) * sum of all previous terms up to a(n-1) for n >= 2.

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A064183 Define a pair of sequences by p(0) = 0, q(0) = p(1) = q(1) = 1, q(n+1) = p(n)*q(n-1), p(n+1) = q(n+1) + q(n) for n > 0; then a(n) = q(n) and A064526(n) = p(n).

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A070231 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)v(k) + v(k)w(k) + w(k)u(k), and w(k+1) = u(k)v(k)*w(k) for k >= 1; then a(n) = u(n).

A070233 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)v(k) + v(k)w(k) + w(k)u(k), and w(k+1) = u(k)v(k)*w(k) for k >= 1; then a(n) = w(n).

A070234 Let u(k), v(k), w(k) satisfy the recursions u(1) = v(1) = w(1) = 1, u(k+1) = u(k) + v(k) + w(k), v(k+1) = u(k)v(k) + v(k)w(k) + w(k)u(k), and w(k+1) = u(k)v(k)*w(k); then a(n) = v(n).