0,3

Number of ordered trees with n+1 edges and height at most 3 (height=number of edges on a maximal path starting at the root). Number of directed column-convex polyominoes of area n+1. Number of nondecreasing Dyck paths of length 2n+2. - Emeric Deutsch (deutsch(AT)duke.poly.edu), Jul 11 2001

Terms for n>1 are the solutions to : 5x^2-4 is a square. - Benoit Cloitre (benoit7848c(AT)orange.fr), Apr 07 2002

a(1) = 1, a(n+1) = smallest Fibonacci number greater than the n-th partial sum. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Oct 21 2002

The fractional part of tau*a(n) decreases monotonically to zero. - Benoit Cloitre (benoit7848c(AT)orange.fr), Feb 01 2003

n such that floor(phi^2*n^2)-floor(phi*n)^2 = 1 where phi=(1+sqrt(5))/2 - Benoit Cloitre (benoit7848c(AT)orange.fr), Mar 16 2003

Number of leftist horizontally convex polyominoes with area n+1.

Number of 31-avoiding words of length n on alphabet {1,2,3} which do not end in 3. (e.g. n=3, we have 111,112,121,122,132,211,212,221,222,232,321,322 and 332). See A028859. - Jon Perry (perry(AT)globalnet.co.uk), Aug 04 2003

Appears to give all solutions >1 to the equation : x^2=ceiling(x*r*floor(x/r)) where r=phi=(1+sqrt(5))/2. - Benoit Cloitre, Feb 24, 2004

a(1) = 1, a(2) = 2, then the least number such that the square of any term is just less than the geometric mean of its neighbors. a(n+1)*a(n-1)> a(n)^2. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Apr 06 2004

All positive integer solutions of Pell equation b(n)^2 - 5*a(n)^2 = -4 together with b(n)=A002878(n), n>=0. - W. Lang (wolfdieter.lang(AT)physik.uni-karlsruhe.de), Aug 31 2004

a(n) = L(n,3), where L is defined as in A108299; see also A002878 for L(n,-3). - Reinhard Zumkeller (reinhard.zumkeller(AT)lhsystems.com), Jun 01 2005

Essentially same as Pisot sequence E(2,5).

Number of permutations of [n+1] avoiding 321 and 3412. E.g. a(3) = 13 because the permutations of [4] avoiding 321 and 3412 are: 1234, 2134, 1324, 1243, 3124, 2314, 2143, 1423, 1342, 4123, 3142, 2413, 2341. - Bridget Eileen Tenner (bridget(AT)math.mit.edu), Aug 15 2005

Number of 1324-avoiding circular permutations on [n+1].

A subset of the Markoff numbers (A002559). - Robert G. Wilson v (rgwv(at)rgwv.com), Oct 05 2005.

(x,y)=(a(n),a(n+1)) are the solutions of x/(yz)+y/(xz)+z/(xy)=3 with z=1. - Floor van Lamoen (fvlamoen(AT)hotmail.com), Nov 29 2001

Number of (s(0), s(1), ..., s(2n)) such that 0 < s(i) < 5 and |s(i) - s(i-1)| = 1 for i = 1,2,....,2n, s(0) = 1, s(2n) = 1. - Herbert Kociemba (kociemba(AT)t-online.de), Jun 10 2004

With interpolated zeros, counts closed walks of length n at the start or end node of P_4. a(n) counts closed walks of length 2n at the start or end node of P_4. The sequence 0,1,0,2,0,5,.. counts walks of length n between the start and second node of P_4. - Paul Barry (pbarry(AT)wit.ie), Jan 26 2005

a(n) = number of ordered trees on n edges containing exactly one non-leaf vertex all of whose children are leaves (every ordered tree must contain at least one such vertex). For example, a(0) = 1 because the root of the tree with no edges is not considered to be a leaf and the condition "all children are leaves" is vacuously satisfied by the root, and a(4) = 13 counts all 14 ordered trees on 4 edges (A000108) except (ignore dots)

|..|

.\/.

which has two such vertices. - David Callan (callan(AT)stat.wisc.edu), Mar 02 2005

Number of directed column-convex polyominoes of area n. Example: a(2)=2 because we have the 1 X 2 and the 2 X 1 rectangles. - Emeric Deutsch (deutsch(AT)duke.poly.edu), Jul 31 2006

Same as the number of Kekule structures in polyphenanthrene in terms of the number of hexagons in extended (1,1)-nanotubes. See Table 1 on page 411 of I. Lukovits and D. Janezic. - Parthasarathy Nambi (PachaNambi(AT)yahoo.com), Aug 22 2006

Number of free generators of degree n of symmetric polynomials in 3-noncommuting variables. - Mike Zabrocki (zabrocki(AT)mathstat.yorku.ca), Oct 24 2006

Inverse: With phi = (sqrt(5) + 1)/2, log_phi((sqrt(5) a(n) + sqrt(5 a(n)^2 - 4))/2) = n for n >= 1. - David W. Cantrell (DWCantrell(AT)sigmaxi.net), Feb 19 2007

Consider a teacher who teaches one student, then he finds he can teach two students while the original student learns to teach a student. And so on with every generation an individual can teach one more student then he could before. a(n) starting at a(2) gives the total number of new students/teachers (see program). - Ben Thurston (benthurston27(AT)yahoo.com), Apr 11 2007

The diophantine equation a(n)=m has a solution (for m>=1) iff ceiling(arsinh(sqr(5)*m/2)/ln(phi)))<>ceiling(arcosh(sqr(5)*m/2)/ln(phi))) where phi is the golden ratio. An equivalent condition is A130255(m)=A130256(m). - Hieronymus Fischer (Hieronymus.Fischer(AT)gmx.de), May 24 2007

a(n+1)= B^(n)(1), n>=0, with compositions of Wythoff's complementary A(n):=A000201(n) and B(n)=A001950(n) sequences. See the W. Lang link under A135817 for the Wythoff representation of numbers (with A as 1 and B as 0 and the argument 1 omitted). E.g. 2=`0`, 5=`00`, 13=`000`,..., in Wythoff code.

E. Barcucci, A. Del Lungo, S. Fezzi and R. Pinzani, Nondecreasing Dyck paths and q-Fibonacci numbers, Discrete Math., 170, 1997, 211-217.

E. Barcucci, R. Pinzani and R. Sprugnoli, Directed column-convex polyominoes by recurrence relations, Lecture Notes in Computer Science, No. 668, Springer, Berlin, 1993, pp. 282-298.

A. T. Benjamin and J. J. Quinn, Proofs that really count: the art of combinatorial proof, M.A.A. 2003, id. 13,15.

S. Brlek, E. Duchi, E. Pergola and S. Rinaldi, On the equivalence problem for succession rules, Discr. Math., 298 (2005), 142-154.

N. G. de Bruijn, D. E. Knuth and S. O. Rice, The average height of planted plane trees, in: Graph Theory and Computing (ed. T. C. Read), Academic Press, New York, 1972, pp. 15-22.

Naiomi T. Cameron and Asamoah Nkwanta, On Some (Pseudo) Involutions in the Riordan Group, Journal of Integer Sequences, Vol. 8 (2005), Article 05.3.7.

E. Deutsch and H. Prodinger, A bijection between directed column-convex polyominoes and ordered trees of height at most three, Theoretical Comp. Science, 307, 2003, 319-325.

A. Gougenheim, About the linear sequence of integers such that each term is the sum of the two preceding, Fib. Quart., 9 (1971), 277-295, 298.

I. Lukovits and D. Janezic, "Enumeration of conjugated circuits in nanotubes", J. Chem. Inf. Comput. Sci., vol. 44 (2004) pp. 410-414.

S. Plouffe, Approximations de S\'{e}ries G\'{e}n\'{e}ratrices et Quelques Conjectures}, Dissertation, Universit\'{e} du Qu\'{e}bec \`{a} Montr\'{e}al, 1992.

S. Rinaldi and D. G. Rogers, Indecomposability: polyominoes and polyomino tilings, Math. Gaz., to appear, 2008.

J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 39.

M. C. Wolf, Symmetric functions of noncommutative elements, Duke Math. J. 2 (1936), 626-637.

F. Yano and H. Yoshida, Some set partition statistics in non-crossing partitions and generating functions, Discr. Math., 307 (2007), 3147-3160.

T. D. Noe, Table of n, a(n) for n=0..200

S. Plouffe, Approximations de S\'{e}ries G\'{e}n\'{e}ratrices et Quelques Conjectures}, Dissertation, Universit\'{e} du Qu\'{e}bec \`{a} Montr\'{e}al, 1992.

S. Plouffe, 1031 Generating Functions and Conjectures, Universit\'{e} du Qu\'{e}bec \`{a} Montr\'{e}al, 1992.

E. Barcucci, A. Del Lungo, R. Pinzani and R. Sprugnoli, La hauteur des polyominos...

N. Bergeron, C. Reutenauer, M. Rosas and M. Zabrocki, Invariants and Coinvariants of the Symmetric Group in Noncommuting Variables, to appear Canad. J. Math.

D. Callan, Pattern avoidance in circular permutations.

Aleksandar Cvetkovic, Predrag Rajkovic and Milos Ivkovic, Catalan Numbers, the Hankel Transform, and Fibonacci Numbers, Journal of Integer Sequences, Vol. 5 (2002), Article 02.1.3

Enrica Duchi, Andrea Frosini, Renzo Pinzani and Simone Rinaldi, A Note on Rational Succession Rules, J. Integer Seqs., Vol. 6, 2003.

J.-P. Ehrmann et al., POLYA003: Integers of the form a/(bc) + b/(ca) + c/(ab).

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 127

I. Jensen, Series exapansions for self-avoiding polygons

Tanya Khovanova, Recursive Sequences

M. Renault, Dissertation

Eric Weisstein's World of Mathematics, Fibonacci Hyperbolic Functions

D. Zeilberger, [math/9801016] Automated counting of LEGO towers

Index entries for sequences related to Chebyshev polynomials.

G.f.: (1-2x)/(1-3x+x^2).

a(n)=3a(n-1)-a(n-2)=a(1-n).

a(n)=(phi^(2n-1)+phi^(1-2n))/sqrt(5) where phi=(1+sqrt(5))/2. - Michael Somos, Oct 28 2002

a(n) = A007598(n-1)+A007598(n) = A000045(n-1)^2+A000045(n)^2 = F(n)^2+F(n+1)^2 - Henry Bottomley (se16(AT)btinternet.com), Feb 09 2001

a(n)=sum(binomial(n+k, 2k), k=0..n). - Len Smiley (smiley(AT)math.uaa.alaska.edu), Dec 09 2001

a(n) ~ (1/5)*sqrt(5)*phi^(2*n+1) - Joe Keane (jgk(AT)jgk.org), May 15 2002

a(1)=1, a(2)=2, a(n+2)=(a(n+1)^2+1)/a(n) - Benoit Cloitre (benoit7848c(AT)orange.fr), Aug 29 2002

a(n) = Sum(k=0, n, C(n, k)*F(k+1)) - Benoit Cloitre (benoit7848c(AT)orange.fr), Sep 03 2002

Let q(n, x)=sum(i=0, n, x^(n-i)*binomial(2*n-i, i)); then q(n, 1)=a(n) (this comment is essentially the same as that of L. Smiley) - Benoit Cloitre (benoit7848c(AT)orange.fr), Nov 10 2002

a(n) = (1/2)*(3*a(n-1)+sqrt(5*a(n-1)^2-4)) - Benoit Cloitre (benoit7848c(AT)orange.fr), Apr 12 2003

Main diagonal of array defined by T(i, 1)=T(1, j)=1, T(i, j)=Max(T(i-1, j)+T(i-1, j-1); T(i-1, j-1)+T(i, j-1)) - Benoit Cloitre (benoit7848c(AT)orange.fr), Aug 05 2003

Hankel transform of A002212. E.g. Det([1, 1, 3;1, 3, 10;3, 10, 36])= 5 . - DELEHAM Philippe (kolotoko(AT)wanadoo.fr), Jan 25 2004

Solutions x > 0 to equation floor(x*r*floor(x/r)) = floor(x/r*floor(x*r)) when r=phi - Benoit Cloitre (benoit7848c(AT)orange.fr), Feb 15 2004

a(n)=sum(i=0, n, binomial(n+i, n-i)) - Jon Perry (perry(AT)globalnet.co.uk), Mar 08 2004

a(n)= S(n, 3) - S(n-1, 3) = T(2*n+1, sqrt(5)/2)/(sqrt(5)/2) with S(n, x)=U(n, x/2), resp. T(n, x), Chebyshev's polynomials of the second, resp. first kind. See triangle A049310, resp. A053120. - W. Lang (wolfdieter.lang(AT)physik.uni-karlsruhe.de), Aug 31 2004

a(n)= ((-1)^n)*S(2*n, I), with the imaginary unit I and S(n, x)=U(n, x/2) Chebyshev's polynomials of the second kind, A049310. - W. Lang (wolfdieter.lang(AT)physik.uni-karlsruhe.de), Aug 31 2004

a(n)=sum_{0<=i_1<=i_2<=n}binomial(i_2, i_1)*binomial(n, i_1+i_2) - Benoit Cloitre (benoit7848c(AT)orange.fr), Oct 14 2004

a( n ) = a( n - 1 ) + SUM[ i = 0 to n - 1 ] a( i ) a( n ) = Fib( 2n + 1 ) SUM[ i = 0 to n - 1 ] a(i) = Fib( 2n ) - Andras Erszegi (erszegi.andras(AT)chello.hu), Jun 28 2005

The i-th term of the sequence is the entry (1, 1) of the i-th power of the 2 by 2 matrix M=((1, 1), (1, 2)). - Simone Severini (ss54(AT)york.ac.uk), Oct 15 2005

a(n-1)=(1/n)*sum_{k=0...n}B(2k)*F(2n-2k)*binomial(2n, 2k) where B(2k) is the (2k)-th Bernoulli number - Benoit Cloitre (benoit7848c(AT)orange.fr), Nov 02 2005

a(n) = A055105(n,1)+A055105(n,2)+A055105(n,3) = A055106(n,1)+A055106(n,2) - Mike Zabrocki (zabrocki(AT)mathstat.yorku.ca), Oct 24 2006

a(n)=2/sqrt(5)*cosh((2n-1)*psi), where psi=ln(phi) and phi=(1+sqrt(5))/2. - Hieronymus Fischer (Hieronymus.Fischer(AT)gmx.de), Apr 24 2007

a(n) = (phi+1)^n - phi*A001906(n) with phi=(1+sqrt(5))/2. - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Nov 22 2007

a(n) = 2*a(n-1)+2*a(n-2)-a(n-3); a(n) = (sqrt(5.0) + 5.0)/10.0*(3.0/2.0 + sqrt(5.0)/2.0)^(n-2) + (-sqrt(5.0) + 5.0)/10.0*(3.0/2.0 - sqrt(5.0)/2.0)^(n-2). - Antonio A. Olivares (olivares14031(AT)yahoo.com), Mar 21 2008

a(3)=13: there are 14 ordered trees with 4 edges; all of them, except the path with 4 edges, have height at most 3.

count:=0; node:=proc(g) global count; for j from 1 to g do for i from 0 to (g-j) do node(j-1); end do; end do; count:=count +1; end proc; - Ben Thurston (benthurston27(AT)yahoo.com), Apr 11 2007

A001519:=-(-1+z)/(1-3*z+z**2); [Conjectured by S. Plouffe in his 1992 dissertation.]

Fibonacci /@ (2Range[29] - 1) (from Robert G. Wilson v (rgwv(at)rgwv.com), Oct 05 2005)

(PARI) a(n)=fibonacci(2*n-1)

(PARI) a(n)=real(quadgen(5)^(2*n))

(PARI) a(n)=subst(poltchebi(n)+poltchebi(n-1), x, 3/2)*2/5

Cf. A000045. First differences of A001906 and of A055588. a(n)= A060920(n, 0).

Row 3 of array A094954.

Equals A001654(n+1) - A001654(n-1), n>0.

Cf. A001653. A122367 is another version.

Cf. A055105, A055106, A055107, A074664, A124292, A124293, A124294, A124295.

Cf. inverse sequences A130255 and A130256.

Adjacent sequences: A001516 A001517 A001518 this_sequence A001520 A001521 A001522

Sequence in context: A027933 A011783 A122367 this_sequence A048575 A099496 A114299

nonn,nice,easy,core

njas

Entry revised by njas, Aug 24 2006