0,3

Number of n-step non-selfintersecting paths starting at (0,0) with steps of types (1,0), (-1,0) or (0,1) [Stanley].

Number of symmetric 2n X 2 or (2n-1) X 2 crossword puzzle grids: all white squares are edge connected; at least 1 white square on every edge of grid; 180 degree rotational symmetry - Erich Friedman (erich.friedman(AT)stetson.edu)

a(n+1) is the number of ways to put molecules on a 2 X n ladder lattice so that the molecules do not touch each other.

Number of (n-1) X 2 binary arrays with a path of adjacent 1's from top row to bottom row. - Ron Hardin (rhhardin(AT)att.net), Mar 16 2002

a(2*n+1) with b(2*n+1) := A000129(2*n+1), n>=0, give all (positive integer) solutions to Pell equation a^2 - 2*b^2 = -1.

a(2*n) with b(2*n) := A000129(2*n), n>=1, give all (positive integer) solutions to Pell equation a^2 - 2*b^2 = +1 (see Emerson reference).

Bisection: a(2*n)= T(n,3)=A001541(n), n>=0 and a(2*n+1)=S(2*n,2*sqrt(2))= A002315(n), n>=0, with T(n,x), resp. S(n,x), Chebyshev's polynomials of the first,resp. second kind. See A053120, resp. A049310.

Binomial transform of A077957. - Paul Barry (pbarry(AT)wit.ie), Feb 25 2003

For n>0, the number of (s(0), s(1), ..., s(n)) such that 0 < s(i) < 4 and |s(i) - s(i-1)| <= 1 for i = 1,2,....,n, s(0) = 2, s(n) = 2. - Herbert Kociemba (kociemba(AT)t-online.de), Jun 02 2004

x satisfying x^2 - 2*y^2 = -+1. Corresponding y is given by A000129(n). - Lekraj Beedassy (blekraj(AT)yahoo.com), Jun 24 2004

For n>1, a(n) corresponds to the longer side of a near right-angled isosceles triangle, one of the equal sides being A000129(n). - Lekraj Beedassy (blekraj(AT)yahoo.com), Aug 06 2004

Exponents of terms in the series F(x,1), where F is determined by the equation F(x,y) = xy + F(x^2*y,x). - Jonathan Sondow (jsondow(AT)alumni.princeton.edu), Dec 18 2004

Number of n-words from the alphabet A={0,1,2} which two neighbors differ by at most 1. - Fung Cheok Yin (cheokyin_restart(AT)yahoo.com.hk), Aug 30 2006

Consider the mapping f(a/b) = (a + 2b)/(a + b). Taking a = b = 1 to start with and carrying out this mapping repeatedly on each new (reduced) rational number gives the following sequence 1/1, 3/2,7/5,17/12,41/29,... converging to 2^(1/2). Sequence contains the numerators. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Mar 22 2003 [Amended by Paul E. Black (paul.black(AT)nist.gov), Dec 18 2006]

a(n) mod 10 = A131707. See A131711. - Paul Curtz (bpcrtz(AT)free.fr), Apr 08 2008

Starting (1, 3, 7, 17,...) = row sums of triangle A140750 - Gary W. Adamson (qntmpkt(AT)yahoo.com), May 26 2008

Prime numerators with an odd index are prime RMS numbers(A140480) and also NSW primes(A088165). [From Ctibor O. Zizka (ctibor.zizka(AT)seznam.cz), Aug 13 2008]

A001333(2*n+1)=A002315(n); A001333(2*n)=A001541(n). [From Ctibor O. Zizka (ctibor.zizka(AT)seznam.cz), Aug 13 2008]

The intermediate convergents to 2^(1/2) begin with 4/3, 10/7, 24/17, 58/41; essentially, numerators=A052542 and denominators=A001333. - Clark Kimberling (ck6(AT)evansville.edu), Aug 26 2008

Contribution from Gary W. Adamson (qntmpkt(AT)yahoo.com), Sep 06 2008: (Start)

Equals right border of triangle A143966. Starting (1, 3, 7,...) equals

INVERT transform of (1, 2, 2, 2,...) and row sums of triangle A143966. (End)

Inverse binomial transform of A006012 ; Hankel transform is := [1, 2, 0, 0, 0, 0, 0, 0, 0, ...] . [From Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Dec 04 2008]

Contribution from Charlie Marion (charliemath(AT)optonline.net), Jan 07 2009: (Start)

In general, denominators, a(k,n) and numerators, b(k,n), of continued

fraction convergents to sqrt((k+1)/k) may be found as follows:

let a(k,0) = 1, a(k,1) = 2k; for n>0, a(k,2n) = 2*a(k,2n-1)+a(k,2n-2)

and a(k,2n+1)=(2k)*a(k,2n)+a(k,2n-1);

let b(k,0) = 1, b(k,1) = 2k+1; for n>0, b(k,2n) = 2*b(k,2n-1)+b(k,2n-2)

and b(k,2n+1)=(2k)*b(k,2n)+b(k,2n-1).

For example, the convergents to sqrt(2/1) start 1/1, 3/2, 7/5,

17/12, 41/29.

In general, if a(k,n) and b(k,n) are the denominators and numerators,

respectively, of continued fraction convergents to sqrt((k+1)/k)

as defined above, then

k*a(k,2n)^2-a(k,2n-1)*a(k,2n+1)=k=k*a(k,2n-2)*a(k,2n)-a(k,2n-1)^2 and

b(k,2n-1)*b(k,2n+1)-k*b(k,2n)^2=k+1=b(k,2n-1)^2-k*b(k,2n-2)*b(k,2n);

for example, if k=1 and n=3, then b(1,n)=a(n+1) and

1*a(1,6)^2-a(1,5)*a(1,7)=1*169^2-70*408=1;

1*a(1,4)*a(1,6)-a(1,5)^2=1*29*169-70^2=1;

b(1,5)*b(1,7)-1*b(1,6)^2=99*577-1*239^2=2;

b(1,5)^2-1*b(1,4)*b(1,6)=99^2-1*41*239=2.

Cf. A000129, A142238-A142239, A153313-153318.

[From Charlie Marion (charliemath(AT)optonline.net), Jan 07 2009]

(End)

Equals row sums of triangle A160756 [From Gary W. Adamson (qntmpkt(AT)yahoo.com), May 25 2009]

Paul Barry, A Catalan Transform and Related Transformations on Integer Sequences, Journal of Integer Sequences, Vol. 8 (2005), Article 05.4.5.

A. H. Beiler, Recreations in the Theory of Numbers. New York: Dover, pp. 122-125, 1964.

F. R. K. Chung and R. L. Graham, Primitive juggling sequences, preprint, 2006.

John Derbyshire, Prime Obsession, Joseph Henry Press, April 2004, see p. 16.

E. I. Emerson, Recurrent sequences in the equation DQ^2=R^2+N, Fib. Quart., 7 (1969), 231-242, see Ex. 1, pp. 237-238.

Reinhardt Euler, The Fibonacci Number of a Grid Graph and a New Class of Integer Sequences, Journal of Integer Sequences, Vol. 8 (2005), Article 05.2.6.

David Garth and Adam Gouge, Affinely Self-Generating Sets and Morphisms, Journal of Integer Sequences, Vol. 10 (2007), Article 07.1.5.

A. F. Horadam, R. P. Loh and A. G. Shannon, Divisibility properties of some Fibonacci-type sequences, pp. 55-64 of Combinatorial Mathematics VI (Armidale 1978), Lect. Notes Math. 748, 1979.

Y. Kong, Ligand binding on ladder lattices, Biophysical Chemistry, Vol. 81 (1999), pp. 7-21.

Kin Y. Li, Math Problem Book I, 2001, p. 24, Problem 159

Munarini, Emanuele, Combinatorial properties of the antichains of a garland. Integers, 9 (2009), 353-374.

I. Niven and H. S. Zuckerman, An Introduction to the Theory of Numbers. 2nd ed., Wiley, NY, 1966, p. 102, Problem 10.

H. Prodinger and R. F. Tichy, Fibonacci numbers of graphs, Fibonacci Quarterly, 20 (1982), 16-21.

B. S. Rao, Heptagonal numbers in the associated Pell sequence ..., Fib. Quarterly, 43 (2005), 302-306.

J. Roberts, Lure of the Integers, Math. Assoc. America, 1992, p. 224.

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).

R. P. Stanley, Enumerative Combinatorics, Volume 1 (1986), p. 203, Example 4.1.2.

A. Tarn, Approximations to certain square roots and the series of numbers connected therewith, Mathematical Questions and Solutions from the Educational Times, 1 (1916), 8-12.

Gy. Tasi et al., Quantum algebraic-combinatoric study of the conformational properties of n-alkanes. II, J. Math. Chemistry, 27, 2000, 191-199 (see p. 193).

V. Thebault, Concerning two classes of remarkable perfect square pairs, Amer. Math. Monthly, 56 (1949), 443-448.

R. C. Tilley et al., The cell growth problem for filaments, Proc. Louisiana Conf. Combinatorics, ed. R. C. Mullin et al., Baton Rouge, 1970, 310-339.

T. D. Noe, Table of n, a(n) for n = 0..500

Joerg Arndt, Fxtbook

C. Banderier and D. Merlini, Lattice paths with an infinite set of jumps, FPSAC02, Melbourne, 2002.

Nick Hobson, Python program for this sequence

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 143

Tanya Khovanova, Recursive Sequences

Kin Y. Li, Problem 159

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.

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

Eric Weisstein's World of Mathematics, Pythagoras's Constant

Eric Weisstein's World of Mathematics, Square Triangular Number

Index entries for "core" sequences

Index entries for sequences related to Chebyshev polynomials.

Index entries for sequences related to linear recurrences with constant coefficients

a(n) = A055642(A125058(n)). - Reinhard Zumkeller, Feb 02 2007

a(n) = 2a(n-1) + a(n-2); a(n) = ( (1-Sqrt[ 2 ])^n + (1+Sqrt[ 2 ])^n) /2.

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

A000129(2n) = 2*A000129(n)*a(n). - John McNamara, Oct 30, 2002

a(n) = ((-i)^n)*T(n, i), with T(n, x) Chebyshev's polynomials of the first kind A053120 and i^2 = -1.

a(n)=a(n-1)+A052542(n-1), n>1. a(n)/A052542(n) converges to sqrt(1/2). - Mario Catalani (mario.catalani(AT)unito.it), Apr 29 2003

E.g.f.: exp(x)cosh(x*sqrt(2)) - Paul Barry (pbarry(AT)wit.ie), May 08 2003

a(n)=sum{k=0..floor(n/2), C(n, 2k)2^k } - Paul Barry (pbarry(AT)wit.ie), May 13 2003

For n >0, a(n )^2 - (1 + (-1)^(n ))/2 =sum_{k=0...n-1}((2k+1)*A001653(n-1-k)); e.g. 17^2-1=288=1*169+3*29+5*5+7*1; 7^2=49=1*29+3*5+5*1 - Charlie Marion (charliem(AT)bestweb.net), Jul 18 2003

A001333(n+2) = A078343(n+1) + A048654(n) - Creighton Dement (creighton.k.dement(AT)uni-oldenburg.de), Jan 19 2005

Conjecture: For prime p, A001333(p) congruent to 1 mod p ( compare with similar comment for A000032 ) - Creighton Dement (creighton.k.dement(AT)uni-oldenburg.de), Oct 11 2005

a(n) = A000129(n)+A000129(n-1) = A001109(n)/A000129(n) = sqrt(A001110(n)/A000129(n)^2) = ceiling(sqrt(A001108(n))) - Henry Bottomley (se16(AT)btinternet.com), Apr 18 2000

Also the first differences of A000129 (the Pell numbers) because A052937(n) = A000129(n+1)+1 - Graeme McRae (g_m(AT)mcraefamily.com), Aug 03 2006

a(n) = Sum_{k,0<=k<=n}A122542(n,k) . - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Oct 08 2006

For another recurrence see A000129.

Starting (1, 3, 7, 17, 41,...), = row sums of triangle A135837. - Gary W. Adamson (qntmpkt(AT)yahoo.com), Dec 01 2007

a(n)=Sum_{k, 0<=k<=n}A098158(n,k)*2^(n-k). - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Dec 26 2007

a(n) = upper left and lower right terms of [1,1; 2,1]^n - Gary W. Adamson (qntmpkt(AT)yahoo.com), Mar 12 2008

(a)=((sqrt2*(1+sqrt2)^n)-(-sqrt2*(1-sqrt2)^n))/sqrt8. Offset 0. a(3)=7 [From Al Hakanson (hawkuu(AT)gmail.com), Apr 11 2009]

Convergents are 1, 3/2, 7/5, 17/12, 41/29, 99/70, 239/169, 577/408, 1393/985, 3363/2378, 8119/5741, 19601/13860, 47321/33461, 114243/80782, ... = A001333/A000129

The 15 3 X 2 crossword grids, with white squares represented by an o:

ooo ooo ooo ooo ooo ooo ooo oo. o.o .oo o.. .o. ..o oo. .oo

ooo oo. o.o .oo o.. .o. ..o ooo ooo ooo ooo ooo ooo .oo oo.

A001333 := proc(n) option remember; if n=0 then 1 elif n=1 then 1 else 2*A001333(n-1)+A001333(n-2) fi end; # version 1

Digits := 50; A001333 := n-> round((1/2)*(1+sqrt(2))^n); # version 2

with(numtheory): cf := cfrac (sqrt(2), 1000): [seq(nthnumer(cf, i), i=0..50)]; # version 3

with(numtheory):cf := cfrac (sin(Pi/4), 100): seq(nthdenom (cf, i), i=0..29 ); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Feb 07 2007

A001333:=-(z+1)/(-1+2*z+z**2); [S. Plouffe in his 1992 dissertation.]

(Maple) a := proc(n) local M; M := (Matrix([[2, 1], [1, 0]])^n); M[2, 1]+M[2, 2] end; seq (a(n), n=0..29); [From Alois P. Heinz (heinz(AT)hs-heilbronn.de), Aug 01 2008]

Insert[Table[Numerator[FromContinuedFraction[ContinuedFraction[Sqrt[2], n]]], {n, 1, 40}], 1, 1] - Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Apr 08 2006

f[n_] := ((1 - Sqrt[2])^n + (1 + Sqrt[2])^n)/2; Table[Simplify@ f@n, {n, 0, 29}] (* Or *)

a[0] = 1; a[1] = 1; a[n_] := a[n] = 2a[n - 1] + a[n - 2]; Table[a@n, {n, 0, 29}] (from Robert G. Wilson v (rgwv(at)rgwv.com), May 02 2006)

a=0; b=1; c=0; lst={b}; Do[c=a+b+c; AppendTo[lst, c]; a=b; b=c, {n, 5!}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Mar 23 2009]

(PARI) a(n)=if(n<0, 0, contfracpnqn(vector(n, i, 1+(i>1)))[1, 1])

sage: from sage.combinat.sloane_functions import recur_gen2 sage: it = recur_gen2(1, 1, 2, 1) sage: [it.next() for i in range(30)] - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 24 2008

(Other) sage: [lucas_number2(n, 2, -1)/2 for n in xrange(0, 30)]# [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Apr 30 2009]

(PARI) { default(realprecision, 2000); for (n=0, 4000, a=contfracpnqn(vector(n, i, 1+(i>1)))[1, 1]; if (a > 10^(10^3 - 6), break); write("b001333.txt", n, " ", a); ); } [From Harry J. Smith (hjsmithh(AT)sbcglobal.net), Jun 12 2009]

For denominators see A000129.

See A040000 for the continued fraction expansion of sqrt(2).

a(n)+a(n+1) = 2 A000129(n+1). 2*a(n) = A002203(n) (companion Pell numbers).

See also A078057 which is the same sequence without the initial 1.

Cf. also A152113.

Row sums of unsigned Chebyshev T-triangle A053120. a(n)= A054458(n, 0) (first column of convolution triangle).

Essentially the same as A078057. Equals A034182(n-1) + 2 and A084128(n)/2^n. First differences of A052937. Partial sums of A052542. Pairwise sums of A048624. Bisection of A002965.

The following sequences (and others) belong to the same family: A001333, A000129, A026150, A002605, A046717, A015518, A084057, A063727, A002533, A002532, A083098, A083099, A083100, A015519.

Second row of the array in A135597.

Cf. A135837, A140750, A143966.

A160756 [From Gary W. Adamson (qntmpkt(AT)yahoo.com), May 25 2009]

Sequence in context: A131056 A077851 A089737 this_sequence A078057 A123335 A089742

Adjacent sequences: A001330 A001331 A001332 this_sequence A001334 A001335 A001336

nonn,cofr,easy,core,nice,frac

N. J. A. Sloane (njas(AT)research.att.com), R. K. Guy

Chebyshev comments from W. Lang (wolfdieter.lang(AT)physik.uni-karlsruhe.de), Jan 10 2003