This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A000245 M2809 N1130 #307 Apr 16 2025 10:58:46
%S A000245 0,1,3,9,28,90,297,1001,3432,11934,41990,149226,534888,1931540,
%T A000245 7020405,25662825,94287120,347993910,1289624490,4796857230,
%U A000245 17902146600,67016296620,251577050010,946844533674,3572042254128,13505406670700,51166197843852,194214400834356
%N A000245 a(n) = 3*(2*n)!/((n+2)!*(n-1)!).
%C A000245 This sequence represents the expected saturation of a binary search tree (or BST) on n nodes times the number of binary search trees on n nodes, or alternatively, the sum of the saturation of all binary search trees on n nodes. - _Marko Riedel_, Jan 24 2002
%C A000245 1->12, 2->123, 3->1234 etc. starting with 1, gives A007001: 1, 12, 12123, 12123121231234... summing the digits gives this sequence. - _Miklos Kristof_, Nov 05 2002
%C A000245 a(n-1) = number of n-th generation vertices in the tree of sequences with unit increase labeled by 2 (cf. _Zoran Sunic_ reference). - _Benoit Cloitre_, Oct 07 2003
%C A000245 With offset 1, number of permutations beginning with 12 and avoiding 32-1.
%C A000245 Number of lattice paths from (0,0) to (n,n) with steps E=(1,0) and N=(0,1) which touch but do not cross the line x-y=1. - _Herbert Kociemba_, May 24 2004
%C A000245 a(n) is the number of Dyck (n+1)-paths that start with UU. For example, a(2)=3 counts UUUDDD, UUDUDD, UUDDUD. - _David Callan_, Dec 08 2004
%C A000245 a(n) is the number of Dyck (n+2)-paths that start with UUDU. For example, a(2)=3 counts UUDUDDUD, UUDUDUDD, UUDUUDDD. - _David Scambler_, Feb 14 2011
%C A000245 Hankel transform is (0,-1,-1,0,1,1,0,-1,-1,0,...). Hankel transform of a(n+1) is (1,0,-1,-1,0,1,1,0,-1,-1,0,...). - _Paul Barry_, Feb 08 2008
%C A000245 Starting with offset 1 = row sums of triangle A154558. - _Gary W. Adamson_, Jan 11 2009
%C A000245 Starting with offset 1 equals INVERT transform of A014137, partial sums of the Catalan numbers: (1, 2, 4, 9, 23, ...). - _Gary W. Adamson_, May 15 2009
%C A000245 With offset 1, a(n) is the binomial transform of the shortened Motzkin numbers: 1, 2, 4, 9, 21, 51, 127, 323, ... (A001006). - Aoife Hennessy (aoife.hennessy(AT)gmail.com), Sep 07 2009
%C A000245 The Catalan sequence convolved with its shifted variant, e.g. a(5) = 90 = (1, 1, 2, 5, 14) dot (42, 14, 5, 2, 1) = (42 + 14 + 10 + 10 + 14 ) = 90. - _Gary W. Adamson_, Nov 22 2011
%C A000245 a(n+2) = A214292(2*n+3,n). - _Reinhard Zumkeller_, Jul 12 2012
%C A000245 With offset 3, a(n) is the number of permutations on {1,2,...,n} that are 123-avoiding, i.e., do not contain a three term monotone subsequence, for which the first ascent is at positions (3,4); see Connolly link. There it is shown in general that the k-th Catalan Convolution is the number of 123-avoiding permutations for which the first ascent is at (k, k+1). (For n=k, the first ascent is defined to be at positions (k,k+1) if the permutation is the decreasing permutation with no ascents.) - _Anant Godbole_, Jan 17 2014
%C A000245 With offset 3, a(n)=number of permutations on {1,2,...,n} that are 123-avoiding and for which the integer n is in the 3rd spot; see Connolly link. For example, there are 297 123-avoiding permutations on n=9 at which the element 9 is in the third spot. - _Anant Godbole_, Jan 17 2014
%C A000245 With offset 1, a(n) is the number of North-East paths from (0,0) to (n+1,n+1) that bounce off y = x to the right exactly once but do not cross y = x vertically. Details can be found in Section 4.4 in Pan and Remmel's link. - _Ran Pan_, Feb 01 2016
%C A000245 The total number of returns (downsteps which end on the line y=0) within the set of all Dyck paths from (0,0) to (2n,0). - _Cheyne Homberger_, Sep 05 2016
%C A000245 a(n) is the number of intervals of the form [s,w] that are distributive (equivalently, modular) lattices in the weak order on S_n, for a fixed simple reflection s. - _Bridget Tenner_, Jan 16 2020
%C A000245 a(n+2) is the number of coalescent histories for a pair (G,S) where G is a gene tree with 3-pseudocaterpillar shape and n leaves, S is a species tree with caterpillar shape and n leaves, and G and S have identical leaf labeling. - _Noah A Rosenberg_, Jun 15 2022
%C A000245 a(n) is the number of parking functions of size n avoiding the patterns 132, 213, and 312. - _Lara Pudwell_, Apr 10 2023
%C A000245 a(n) is the number of parking functions of size n avoiding the patterns 123 and 213. - _Lara Pudwell_, Apr 10 2023
%D A000245 Pierre de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 11, coefficients of P_3(z).
%D A000245 Ki Hang Kim, Douglas G. Rogers and Fred W. Roush, Similarity relations and semiorders. Proceedings of the Tenth Southeastern Conference on Combinatorics, Graph Theory and Computing (Florida Atlantic Univ., Boca Raton, Fla., 1979), pp. 577-594, Congress. Numer., XXIII-XXIV, Utilitas Math., Winnipeg, Man., 1979. MR0561081 (81i:05013)
%D A000245 C. Krishnamachary and M. Bheemasena Rao, Determinants whose elements are Eulerian, prepared Bernoullian and other numbers, J. Indian Math. Soc., Vol. 14 (1922), pp. 55-62, 122-138 and 143-146.
%D A000245 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A000245 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A000245 T. D. Noe, <a href="/A000245/b000245.txt">Table of n, a(n) for n = 0..100</a>
%H A000245 Marco Abrate, Stefano Barbero, Umberto Cerruti and Nadir Murru, <a href="http://dx.doi.org/10.1016/j.disc.2014.06.026">Colored compositions, Invert operator and elegant compositions with the "black tie"</a>, Discrete Math. 335 (2014), 1-7. MR3248794
%H A000245 Ayomikun Adeniran and Lara Pudwell, <a href="https://doi.org/10.54550/ECA2023V3S3R17">Pattern avoidance in parking functions</a>, Enumer. Comb. Appl. 3:3 (2023), Article S2R17.
%H A000245 Anwar Al Ghabra, K. Gopala Krishna, Patrick Labelle, and Vasilisa Shramchenko, <a href="https://arxiv.org/abs/2301.09765">Enumeration of multi-rooted plane trees</a>, arXiv:2301.09765 [math.CO], 2023.
%H A000245 F. S. Al-Kharousi, A. Umar, and M. M. Zubairu, <a href="https://arxiv.org/abs/2501.00285">On injective partial Catalan monoids</a>, arXiv:2501.00285 [math.GR], 2024. See p. 2.
%H A000245 Egor Alimpiev and Noah A Rosenberg, <a href="https://arxiv.org/abs/2103.13464">Enumeration of coalescent histories for caterpillar species trees and p-pseudocaterpillar gene trees</a>, arXiv:2103.13464 [qbio.PE], 2021; <a href="https://doi.org/10.1016/j.aam.2021.102265">Adv. Appl. Math. 131 (2021), 102265</a>.
%H A000245 Jean-Luc Baril, <a href="https://doi.org/10.37236/665">Classical sequences revisited with permutations avoiding dotted pattern</a>, Electronic Journal of Combinatorics, 18 (2011), #P178.
%H A000245 Jean-Luc Baril and Sergey Kirgizov, <a href="http://jl.baril.u-bourgogne.fr/Stirling.pdf">The pure descent statistic on permutations</a>, Preprint, 2016.
%H A000245 Jean-Luc Baril and Helmut Prodinger, <a href="https://arxiv.org/abs/2205.01383">Enumeration of partial Lukasiewicz paths</a>, arXiv:2205.01383 [math.CO], 2022.
%H A000245 Paul Barry, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL8/Barry/barry84.html">A Catalan Transform and Related Transformations on Integer Sequences</a>, Journal of Integer Sequences, Vol. 8 (2005), Article 05.4.5.
%H A000245 Paul Barry, <a href="https://arxiv.org/abs/1807.05794">Riordan Pseudo-Involutions, Continued Fractions and Somos 4 Sequences</a>, arXiv:1807.05794 [math.CO], 2018.
%H A000245 Paul Barry, <a href="https://arxiv.org/abs/2004.04577">On a Central Transform of Integer Sequences</a>, arXiv:2004.04577 [math.CO], 2020.
%H A000245 David Callan, <a href="https://arxiv.org/abs/math/0211380">A recursive bijective approach to counting permutations containing 3-letter patterns</a>, arXiv:math/0211380 [math.CO], Nov 25 2002.
%H A000245 Reinis Cirpons, James East, and James D. Mitchell, <a href="https://arxiv.org/abs/2411.14693">Transformation representations of diagram monoids</a>, arXiv:2411.14693 [math.RA], 2024. See pp. 3, 33.
%H A000245 S. Connolly, Z. Gabor and A. Godbole, <a href="http://arxiv.org/abs/1401.2691"> The location of the first ascent in a 123-avoiding permutation</a>, arXiv:1401.2691 [math.CO], 2014.
%H A000245 S. J. Cyvin and I. Gutman, <a href="https://doi.org/10.1007/978-3-662-00892-8">Kekulé structures in benzenoid hydrocarbons</a>, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (see p. 196).
%H A000245 Olivier Danvy, <a href="https://arxiv.org/abs/2412.03127">Summa Summarum: Moessner's Theorem without Dynamic Programming</a>, arXiv:2412.03127 [cs.DM], 2024. See p. 31.
%H A000245 Dennis E. Davenport, Lara K. Pudwell, Louis W. Shapiro and Leon C. Woodson, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL18/Davenport/dav3.html">The Boundary of Ordered Trees</a>, Journal of Integer Sequences, Vol. 18 (2015), Article 15.5.8.
%H A000245 Filippo Disanto, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL18/Disanto/disanto5.html">Some Statistics on the Hypercubes of Catalan Permutations</a>, Journal of Integer Sequences, Vol. 18 (2015), Article 15.2.2.
%H A000245 Filippo Disanto and Thomas Wiehe, <a href="http://arxiv.org/abs/1210.6908">Some instances of a sub-permutation problem on pattern avoiding permutations</a>, arXiv preprint arXiv:1210.6908 [math.CO], 2012-2014.
%H A000245 Filippo Disanto and Thomas Wiehe, <a href="http://dx.doi.org/10.1016/j.disc.2014.06.028">On the sub-permutations of pattern avoiding permutations</a>, Discrete Math., 337 (2014), 127-141.
%H A000245 Manuel Flores, Yuta Kimura and Baptiste Rognerud, <a href="https://arxiv.org/abs/2004.04726">Combinatorics of quasi-hereditary structures</a>, arXiv:2004.04726 [math.RT], 2020.
%H A000245 Sela Fried and Toufik Mansour, <a href="https://arxiv.org/abs/2405.06962">Counting r X s rectangles in (Catalan) words</a>, arXiv:2405.06962 [math.CO], 2024. See p. 11.
%H A000245 Alice L.L. Gao, Sergey Kitaev, and Philip B. Zhang, <a href="https://arxiv.org/abs/1605.05490">On pattern avoiding indecomposable permutations</a>, arXiv:1605.05490 [math.CO], 2016.
%H A000245 N. S. S. Gu, N. Y. Li and T. Mansour, <a href="http://dx.doi.org/10.1016/j.disc.2007.04.007">2-Binary trees: bijections and related issues</a>, Discr. Math., 308 (2008), 1209-1221.
%H A000245 R. K. Guy, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL3/GUY/catwalks.html">Catwalks, Sandsteps and Pascal Pyramids</a>, J. Integer Seqs., Vol. 3 (2000), #00.1.6
%H A000245 Guo-Niu Han, <a href="http://www-irma.u-strasbg.fr/~guoniu/papers/p77puzzle.pdf">Enumeration of Standard Puzzles</a> [broken link]
%H A000245 Guo-Niu Han, <a href="/A196265/a196265.pdf">Enumeration of Standard Puzzles</a> [Cached copy]
%H A000245 V. E. Hoggatt, Jr. and M. Bicknell, <a href="https://web.archive.org/web/2024*/https://www.fq.math.ca/Scanned/14-5/hoggatt1.pdf">Catalan and related sequences arising from inverses of Pascal's triangle matrices</a>, Fib. Quart., 14 (1976), 395-405.
%H A000245 Sergey Kitaev, <a href="https://www.emis.de/journals/SLC/wpapers/s48kitaev.html">Generalized pattern avoidance with additional restrictions</a>, Sem. Lothar. Combinat. B48e (2003). See also <a href="https://arxiv.org/abs/math/0205215">arXiv:0205215</a> [math.CO], 2002.
%H A000245 Sergey Kitaev and Toufik Mansour, <a href="https://arxiv.org/abs/math/0205182">Simultaneous avoidance of generalized patterns</a>, arXiv:math/0205182 [math.CO], 2002.
%H A000245 C. Krishnamachary and M. Bheemasena Rao, <a href="/A000108/a000108_10.pdf">Determinants whose elements are Eulerian, prepared Bernoullian and other numbers</a>, J. Indian Math. Soc., 14 (1922), 55-62, 122-138 and 143-146. [Annotated scanned copy]
%H A000245 D. Nečas, <a href="https://doi.org/10.1088/1402-4896/adc044">Elementary models of low-pressure plasma polymerisation into nanofibrous mats</a>, Phys. Scr. (2025) Vol. 100, 055601. See p. 10.
%H A000245 Ran Pan and Jeffrey B. Remmel, <a href="http://arxiv.org/abs/1601.07988">Paired patterns in lattice paths</a>, arXiv:1601.07988 [math.CO], 2016.
%H A000245 A. Papoulis, <a href="/A000108/a000108_8.pdf">A new method of inversion of the Laplace transform</a>, Quart. Appl. Math 14 (1957), 405-414. [Annotated scan of selected pages]
%H A000245 A. Papoulis, <a href="http://www.jstor.org/stable/43636019">A new method of inversion of the Laplace transform</a>, Quart. Applied Math. 14 (1956), 405ff.
%H A000245 J.-B. Priez and A. Virmaux, <a href="http://arxiv.org/abs/1411.4161">Non-commutative Frobenius characteristic of generalized parking functions: Application to enumeration</a>, arXiv:1411.4161 [math.CO], 2014-2015.
%H A000245 Jocelyn Quaintance and Harris Kwong, <a href="http://www.emis.de/journals/INTEGERS/papers/n29/n29.Abstract.html">A combinatorial interpretation of the Catalan and Bell number difference tables</a>, Integers, 13 (2013), #A29.
%H A000245 J. Riordan, <a href="/A000262/a000262_1.pdf">Letter to N. J. A. Sloane, Nov 10 1970</a>
%H A000245 J. Riordan, <a href="http://www.jstor.org/stable/2005477">The distribution of crossings of chords joining pairs of 2n points on a circle</a>, Math. Comp., 29 (1975), 215-222.
%H A000245 Zoran Sunic, <a href="http://www.combinatorics.org/ojs/index.php/eljc/article/view/v10i1n5">Self describing sequences and the Catalan family tree</a>, Elect. J. Combin., 10 (No. 1, 2003).
%H A000245 Murray Tannock, <a href="http://hdl.handle.net/1946/25589">Equivalence classes of mesh patterns with a dominating pattern</a>, MSc Thesis, Reykjavik Univ., May 2016.
%H A000245 S. J. Tedford, <a href="http://www.emis.de/journals/INTEGERS/papers/l3/l3.Abstract.html">Combinatorial interpretations of convolutions of the Catalan numbers</a>, Integers 11 (2011) #A3.
%H A000245 Bridget E. Tenner, <a href="https://arxiv.org/abs/2001.05011">Interval structures in the Bruhat and weak orders</a>, arXiv:2001.05011 [math.CO], 2020.
%H A000245 Qi Wang, <a href="https://arxiv.org/abs/1910.01937">Tau-tilting finite simply connected algebras</a>, arXiv:1910.01937 [math.RT], 2019.
%F A000245 a(n) = A000108(n+1) - A000108(n).
%F A000245 G.f.: x*(c(x))^3 = (-1+(1-x)*c(x))/x, c(x) = g.f. for Catalan numbers. Also a(n) = 3*n*Catalan(n)/(n+2). - _Wolfdieter Lang_
%F A000245 For n > 1, a(n) = 3a(n-1) + Sum[a(k)*a(n-k-2), k=1,...,n-3]. - _John W. Layman_, Dec 13 2002; proved by _Michael Somos_, Jul 05 2003
%F A000245 G.f. is A(x) = C(x)*(1-x)/x-1/x = x(1+x*C(x)^2)*C(x)^2 where C(x) is g.f. for Catalan numbers, A000108.
%F A000245 G.f. satisfies x^2*A(x)^2 + (3*x-1)*A(x) + x = 0.
%F A000245 Series reversion of g.f. A(x) is -A(-x). - _Michael Somos_, Jan 21 2004
%F A000245 a(n+1) = Sum_{i+j+k=n} C(i)C(j)C(k) with i, j, k >= 0 and where C(k) denotes the k-th Catalan number. - _Benoit Cloitre_, Nov 09 2003
%F A000245 An inverse Chebyshev transform of x^2. - _Paul Barry_, Oct 13 2004
%F A000245 The sequence is 0, 0, 1, 0, 3, 0, 9, 0, ... with zeros restored. Second binomial transform of (-1)^n*A005322(n). The g.f. is transformed to x^2 under the Chebyshev transformation A(x)->(1/(1+x^2))A(x/(1+x^2)). For a sequence b(n), this corresponds to taking Sum_{k=0..floor(n/2)} C(n-k, k)(-1)^k*b(n-2k), or Sum_{k=0..n} C((n+k)/2, k)*b(k)*(-1)^((n-k)/2)*(1+(-1)^(n-k))/2. - _Paul Barry_, Oct 13 2004
%F A000245 G.f.: (c(x^2)*(1-x^2)-1)/x^2, c(x) the g.f. of A000108; a(n) = Sum_{k=0..n} (k+1)*C(n, (n-k)/2)*(-1)^k*(C(2,k)-2*C(1,k)+C(0, k))*(1+(-1)^(n-k))/(n+k+2). - _Paul Barry_, Oct 13 2004
%F A000245 a(n) = Sum_{k=0..n} binomial(n,k)*2^(n-k)*(-1)^(k+1)*binomial(k, floor((k-1)/2)). - _Paul Barry_, Feb 16 2006
%F A000245 E.g.f.: exp(2*x)*(Bessel_I(1,2x) - Bessel_I(2,2*x)). - _Paul Barry_, Jun 04 2007
%F A000245 a(n) = (1/Pi)*Integral_{x=0..4} x^n*(x-1)*sqrt(x*(4-x))/(2*x). - _Paul Barry_, Feb 08 2008
%F A000245 D-finite with recurrence: For n > 1, a(n+1) = 2*(2n+1)*(n+1)*a(n)/((n+3)*n). - _Sean A. Irvine_, Dec 09 2009
%F A000245 Let A be the Toeplitz matrix of order n defined by: A[i,i-1]=-1, A[i,j] = Catalan(j-i), (i<=j), and A[i,j] = 0, otherwise. Then, for n >= 2, a(n-1) = (-1)^(n-2)*coeff(charpoly(A,x),x^2). - _Milan Janjic_, Jul 08 2010
%F A000245 a(n) = sum of top row terms of M^(n-1), M = an infinite square production matrix as follows:
%F A000245 2, 1, 0, 0, 0, 0, ...
%F A000245 1, 1, 1, 0, 0, 0, ...
%F A000245 1, 1, 1, 1, 0, 0, ...
%F A000245 1, 1, 1, 1, 1, 0, ...
%F A000245 1, 1, 1, 1, 1, 1, ...
%F A000245 ...
%F A000245 - _Gary W. Adamson_, Jul 14 2011
%F A000245 E.g.f.: exp(2*x)*(BesselI(2,2*x)) = Q(0) - 1 where Q(k) = 1 - 2*x/(k + 1 - 3*((k+1)^2)/((k^2) + 8*k + 9 - (k+2)*((k+3)^2)*(2*k+3)/((k+3)*(2*k+3) - 3*(k+1)/Q(k+1)))); (continued fraction). - _Sergei N. Gladkovskii_, Dec 05 2011
%F A000245 a(n) = -binomial(2*n,n)/(n+1)*hypergeom([-1,n+1/2],[n+2],4). - _Peter Luschny_, Aug 15 2012
%F A000245 a(n) = Sum_{i=0..n-1} C(i)*C(n-i), where C(i) denotes the i-th Catalan number. - _Dmitry Kruchinin_, Mar 02 2013
%F A000245 a(n) = binomial(2*n-1, n) - binomial(2*n-1, n-3). - _Johannes W. Meijer_, Jul 31 2013
%F A000245 a(n) ~ 3*4^n/(n^(3/2)*sqrt(Pi)). - _Vaclav Kotesovec_, Feb 26 2016
%F A000245 a(n) = ((-1)^n/(n+1))*Sum_{i=0..n-1} (-1)^(i+1)*(n+1-i)*binomial(2*n+2,i), n>=0. - _Taras Goy_, Aug 09 2018
%F A000245 From _Amiram Eldar_, Jan 02 2022: (Start)
%F A000245 Sum_{n>=1} 1/a(n) = 14*Pi/(27*sqrt(3)) + 5/9.
%F A000245 Sum_{n>=1} (-1)^(n+1)/a(n) = 164*log(phi)/(75*sqrt(5)) + 7/25, where phi is the golden ratio (A001622). (End)
%F A000245 a(n) = 3*Sum_{k = 0..n-2} (-1)^k * binomial(2*n-k-1, n+1)*binomial(n+1, k)/(k + 1) for n >= 2. - _Peter Bala_, Sep 02 2024
%F A000245 a(n) = (3*n/(n+2))*A000108(n). - _Taras Goy_, Dec 20 2024
%p A000245 A000245 := n -> 3*binomial(2*n, n-1)/(n+2);
%p A000245 seq(A000245(n), n=0..27);
%t A000245 Table[3(2n)!/((n+2)!(n-1)!),{n,0,30}] (* or *) Table[3*Binomial[2n,n-1]/(n+2),{n,0,30}] (* or *) Differences[CatalanNumber[Range[0,31]]] (* _Harvey P. Dale_, Jul 13 2011 *)
%o A000245 (PARI) a(n)=if(n<1,0,3*(2*n)!/(n+2)!/(n-1)!)
%o A000245 (Sage) [catalan_number(i+1) - catalan_number(i) for i in range(28)] # _Zerinvary Lajos_, May 17 2009
%o A000245 (Sage)
%o A000245 def A000245_list(n) :
%o A000245 D = [0]*(n+1); D[1] = 1
%o A000245 b = False; h = 1; R = []
%o A000245 for i in range(2*n-1) :
%o A000245 if b :
%o A000245 for k in range(h,0,-1) : D[k] += D[k-1]
%o A000245 h += 1; R.append(D[2])
%o A000245 else :
%o A000245 for k in range(1,h, 1) : D[k] += D[k+1]
%o A000245 b = not b
%o A000245 return R
%o A000245 A000245_list(29) # _Peter Luschny_, Jun 03 2012
%o A000245 (Magma) [0] cat [3*Factorial(2*n)/(Factorial(n+2)*Factorial(n-1)): n in [1..30]]; // _Vincenzo Librandi_, Feb 15 2016
%o A000245 (GAP) Concatenation([0],List([1..30],n->3*Factorial(2*n)/(Factorial(n+2)*Factorial(n-1)))); # _Muniru A Asiru_, Aug 09 2018
%Y A000245 First differences of Catalan numbers A000108.
%Y A000245 T(n, n+3) for n=0, 1, 2, ..., array T as in A047072.
%Y A000245 Cf. A099364.
%Y A000245 A diagonal of any of the essentially equivalent arrays A009766, A030237, A033184, A059365, A099039, A106566, A130020, A047072.
%Y A000245 Cf. A002057, A000344, A003517, A000588, A003518, A003519, A001392, A001622.
%Y A000245 Cf. A154558, A014137.
%Y A000245 Column k=1 of A067323.
%K A000245 nonn,easy,nice
%O A000245 0,3
%A A000245 _N. J. A. Sloane_
%E A000245 I changed the description and added an initial 0, to make this coincide with the first differences of the Catalan numbers A000108. Some of the other lines will need to be changed as a result. - _N. J. A. Sloane_, Oct 31 2003