

A000292


Tetrahedral (or triangular pyramidal) numbers: a(n) = C(n+2,3) = n*(n+1)*(n+2)/6.
(Formerly M3382 N1363)


813



0, 1, 4, 10, 20, 35, 56, 84, 120, 165, 220, 286, 364, 455, 560, 680, 816, 969, 1140, 1330, 1540, 1771, 2024, 2300, 2600, 2925, 3276, 3654, 4060, 4495, 4960, 5456, 5984, 6545, 7140, 7770, 8436, 9139, 9880, 10660, 11480, 12341, 13244, 14190, 15180
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OFFSET

0,3


COMMENTS

a(n) is the number of balls in a triangular pyramid in which each edge contains n balls.
One of the 5 Platonic polyhedral (tetrahedral, cube, octahedral, dodecahedral and icosahedral) numbers (cf. A053012).
Also (1/6)*(n^3 + 3*n^2 + 2*n) is the number of ways to color the vertices of a triangle using <= n colors, allowing rotations and reflections. Group is the dihedral group D_6 with cycle index (x1^3 + 2*x3 + 3*x1*x2)/6.
Also the convolution of the natural numbers with themselves.  Felix Goldberg (felixg(AT)tx.technion.ac.il), Feb 01 2001
Connected with the Eulerian numbers (1, 4, 1) via 1*a(x2) + 4*a(x1) + 1*a(x) = x^3.  Gottfried Helms, Apr 15 2002
a(n) is sum of all the possible products p*q where (p,q) are ordered pairs and p + q = n + 1. E.g., a(5) = 5 + 8 + 9 + 8 + 5 = 35.  Amarnath Murthy, May 29 2003
Number of labeled graphs on n+3 nodes that are triangles.  Jon Perry, Jun 14 2003
Number of permutations of n+3 which have exactly 1 descent and avoid the pattern 1324.  Mike Zabrocki, Nov 05 2004
Schlaefli symbol for this polyhedron: {3,3}.
Transform of n^2 under the Riordan array (1/(1x^2), x).  Paul Barry, Apr 16 2005
a(n) is a perfect square only for n = {1, 2, 48}. E.g., a(48) = 19600 = 140^2.  Alexander Adamchuk, Nov 24 2006
a(n+1) is the number of terms in the expansion of (a_1 + a_2 + a_3 + a_4)^n.  Sergio Falcon, Feb 12 2007 [Corrected by Graeme McRae, Aug 28 2007]
a(n+1) is the number of terms in the complete homogeneous symmetric polynomial of degree n in 3 variables.  Richard Barnes, Sep 06 2017
This is also the average "permutation entropy", sum((pi(n)n)^2)/n!, over the set of all possible n! permutations pi.  Jeff Boscole (jazzerciser(AT)hotmail.com), Mar 20 2007
a(n) = (d/dx)(S(n, x), x)_{x = 2}. First derivative of Chebyshev Spolynomials evaluated at x = 2. See A049310.  Wolfdieter Lang, Apr 04 2007
If X is an nset and Y a fixed (n1)subset of X then a(n2) is equal to the number of 3subsets of X intersecting Y.  Milan Janjic, Aug 15 2007
a(n) is the number of gifts received from the lyricist's true love up to and including day n in the song "The Twelve Days of Christmas". a(12) = 364, almost the number of days in the year.  Bernard Hill (bernard(AT)braeburn.co.uk), Dec 05 2008
Sequence of the absolute values of the z^1 coefficients of the polynomials in the GF2 denominators of A156925. See A157703 for background information.  Johannes W. Meijer, Mar 07 2009
This is a 'Matryoshka doll' sequence with alpha=0, the multiplicative counterpart is A000178: seq(add(add(i,i=alpha..k),k=alpha..n),n=alpha..50).  Peter Luschny, Jul 14 2009
a(n) is the number of nondecreasing, threeelement permutations of n distinct numbers.  Samuel Savitz, Sep 12 2009
a(n+4) is the number of different partitions of number n on sum of 4 elements. E.g., a(6) = a(2+4) because we have 10 different partitions of 2 on sum of 4 elements 2 = 2 + 0 + 0 + 0 = 1 + 1 + 0 + 0 = 0 + 2 + 0 + 0 = 1 + 0 + 1 + 0 = 0 + 1 + 1 + 0 = 0 + 0 + 2 + 0 = 1 + 0 + 0 + 1 = 0 + 1 + 0 + 1 = 0 + 0 + 1 + 1 = 0 + 0 + 0 + 2.  Artur Jasinski, Nov 30 2009
a(n) corresponds to the total number of steps to memorize n verses by the technique described in A173564.  Ibrahima Faye (ifaye2001(AT)yahoo.fr), Feb 22 2010
The number of (n+2)bit numbers which contain two runs of 1's in their binary expansion.  Vladimir Shevelev, Jul 30 2010
a(n) is also, starting at the second term, the number of triangles formed in ngons by intersecting diagonals with three diagonal endpoints (see the first column of the table in Sommars link).  Alexandre Wajnberg, Aug 21 2010
Column sums of:
1 4 9 16 25...
1 4 9...
1...
..............

1 4 10 20 35...
The Ca3, Ca4, Gi3 and Gi4 triangle sums (see A180662 for their definitions) of the ConnellPol triangle A159797 are linear sums of shifted versions of the duplicated tetrahedral numbers, e.g., Gi3(n) = 17*a(n) + 19*a(n1) and Gi4(n) = 5*a(n) + a(n1).
Furthermore the Kn3, Kn4, Ca3, Ca4, Gi3 and Gi4 triangle sums of the Connell sequence A001614 as a triangle are also linear sums of shifted versions of the sequence given above. (End)
a(n2)=N_0(n), n >= 1, with a(1):=0, is the number of vertices of n planes in generic position in threedimensional space. See a comment under A000125 for general arrangement. Comment to Arnold's problem 199011, see the Arnold reference, p. 506.  Wolfdieter Lang, May 27 2011
We consider optimal proper vertex colorings of a graph G. Assume that the labeling, i.e., coloring starts with 1. By optimality we mean that the maximum label used is the minimum of the maximum integer label used across all possible labelings of G. Let S=Sum of the differences l(v)  l(u), the sum being over all edges uv of G and l(w) is the label associated with a vertex w of G. We say G admits unique labeling if all possible labelings of G is Sinvariant and yields the same integer partition of S. With an offset this sequence gives the Svalues for the complete graph on n vertices, n = 2, 3, ... .  K.V.Iyer, Jul 08 2011
Central term of commutator of transverse Virasoro operators in 4D case for relativistic quantum open strings (ref. Zwiebach).  Tom Copeland, Sep 13 2011
Appears as a coefficient of a SturmLiouville operator in the Ovsienko reference on page 43.  Tom Copeland, Sep 13 2011
Regarding the second comment above by Amarnath Murthy (May 29 2003), see A181118 which gives the sequence of ordered pairs.  L. Edson Jeffery, Dec 17 2011
The dimension of the space spanned by the 3form v[ijk] that couples to M2brane worldsheets wrapping 3cycles inside tori (ref. Green, Miller, Vanhove eq. 3.9).  Stephen Crowley, Jan 05 2012
a(n+1) is the number of 2 X 2 matrices with all terms in {0, 1, ..., n} and (sum of terms) = n. Also, a(n+1) is the number of 2 X 2 matrices with all terms in {0, 1, ..., n} and (sum of terms) = 3n.  Clark Kimberling, Mar 19 2012
Using n + 4 consecutive triangular numbers t(1), t(2), ..., t(n+4), where n is the nth term of this sequence, create a polygon by connecting points (t(1), (t(2)) to (t(2), t(3)), (t(2), t(3)) to (t(3), t(4)), ..., (t(1), t(2)) to (t(n+3), t(n+4)). The area of this polygon will be onehalf of each term in this sequence.  J. M. Bergot, May 05 2012
Pisano period lengths: 1, 4, 9, 8, 5, 36, 7, 16, 27, 20, 11, 72, 13, 28, 45, 32, 17,108, 19, 40, ... . (The Pisano sequence modulo m is the auxiliary sequence p(n) = a(n) mod m, n >= 1, for some m. p(n) is periodic for all sequences with rational g.f., like this one, and others. The lengths of the period of p(n) are quoted here for m>=1.)  R. J. Mathar, Aug 10 2012
a(n) is the maximum possible number of rooted triples consistent with any phylogenetic tree (level0 phylogenetic network) containing exactly n+2 leaves.  Jesper Jansson, Sep 10 2012
For n > 0, the digital roots of this sequence A010888(a(n)) form the purely periodic 27cycle {1, 4, 1, 2, 8, 2, 3, 3, 3, 4, 7, 4, 5, 2, 5, 6, 6, 6, 7, 1, 7, 8, 5, 8, 9, 9, 9}, which just rephrases the Pisano period length above.  Ant King, Oct 18 2012
a(n) is the number of functions f from {1, 2, 3} to {1, 2, ..., n + 4} such that f(1) + 1 < f(2) and f(2) + 1 < f(3).  Dennis P. Walsh, Nov 27 2012
a(n) is the Szeged index of the path graph with n+1 vertices; see the Diudea et al. reference, p. 155, Eq. (5.8).  Emeric Deutsch, Aug 01 2013
Also the number of permutations of length n that can be sorted by a single block transposition.  Vincent Vatter, Aug 21 2013
a(n) is the 3 X 3 matrix determinant
 C(n,1) C(n,2) C(n,3) 
 C(n+1,1) C(n+1,2) C(n+1,3) 
 C(n+2,1) C(n+2,2) C(n+2,3) 
(End)
In physics, a(n)/2 is the trace of the spin operator S_z^2 for a particle with spin S=n/2. For example, when S=3/2, the S_z eigenvalues are 3/2, 1/2, +1/2, +3/2 and the sum of their squares is 10/2 = a(3)/2.  Stanislav Sykora, Nov 06 2013
a(n+1) = (n+1)*(n+2)*(n+3)/6 is also the dimension of the Hilbert space of homogeneous polynomials of degree n.  L. Edson Jeffery, Dec 12 2013
For n >= 4, a(n3) is the number of permutations of 1,2...,n with the distribution of up (1)  down (0) elements 0...0111 (n4 zeros), or, equivalently, a(n3) is updown coefficient {n,7} (see comment in A060351).  Vladimir Shevelev, Feb 15 2014
a(n) is onehalf the area of the region created by plotting the points (n^2,(n+1)^2). A line connects points (n^2,(n+1)^2) and ((n+1)^2, (n+2)^2) and a line is drawn from (0,1) to each increasing point. From (0,1) to (4,9) the area is 2; from (0,1) to (9,16) the area is 8; further areas are 20,40,70,...,2*a(n).  J. M. Bergot, May 29 2014
Beukers and Top prove that no tetrahedral number > 1 equals a square pyramidal number A000330.  Jonathan Sondow, Jun 21 2014
a(n+1) is for n >= 1 the number of nondecreasing nletter words over the alphabet [4] = {1, 2, 3, 4} (or any other four distinct numbers). a(2+1) = 10 from the words 11, 22, 33, 44, 12, 13, 14, 23, 24, 34; which is also the maximal number of distinct elements in a symmetric 4 X 4 matrix. Inspired by the Jul 20 2014 comment by R. J. Cano on A000582.  Wolfdieter Lang, Jul 29 2014
Degree of the qpolynomial counting the orbits of plane partitions under the action of the symmetric group S3. Orbitcounting generating function is product_{i <= j <= k <= n} ( (1  q^(i + j + k  1))/(1  q^(i + j + k  2)) ). See qTSPP reference.  Olivier Gérard, Feb 25 2015
If n is even then a(n) = Sum_{k=1..n/2} (2k)^2. If n is odd then a(n) = Sum_{k=0..(n1)/2} (1+2k)^2. This can be illustrated as stacking boxes inside a square pyramid on plateaus of edge lengths 2k or 2k+1, respectively. The largest k are the 2k X 2k or (2k+1) X (2k+1) base.  R. K. Guy, Feb 26 2015
Draw n lines in general position in the plane. Any three define a triangle, so in all we see C(n,3) = a(n2) triangles (6 lines produce 4 triangles, and so on).  Terry Stickels, Jul 21 2015
a(n2) = fallfac(n,3)/3!, n >= 3, is also the number of independent components of an antisymmetric tensor of rank 3 and dimension n. Here fallfac is the falling factorial.  Wolfdieter Lang, Dec 10 2015
Number of compositions (ordered partitions) of n+3 into exactly 4 parts.  Juergen Will, Jan 02 2016
Number of weak compositions (ordered weak partitions) of n1 into exactly 4 parts.  Juergen Will, Jan 02 2016
For n >= 2 gives the number of multiplications of two nonzero matrix elements in calculating the product of two upper n X n triangular matrices.  John M. Coffey, Jun 23 2016
Terms a(4n+1), n >= 0, are odd, all others are even. The 2adic valuation of the subsequence of every other term, a(2n+1), n >= 0, yields the ruler sequence A007814. Sequence A275019 gives the 2adic valuation of a(n).  M. F. Hasler, Dec 05 2016
Does not satisfy Benford's law [Ross, 2012].  N. J. A. Sloane, Feb 12 2017
C(n+2,3) is the number of ways to select 1 triple among n+2 objects, thus a(n) is the coefficient of x1^(n1)*x3 in exponential Bell polynomial B_{n+2}(x1,x2,...), hence its link with A050534 and A001296 (see formula).  Cyril Damamme, Feb 26 2018
a(n) is also the number of 3cycles in the (n+4)path complement graph.  Eric W. Weisstein, Apr 11 2018
a(n) is the general number of all geodetic graphs of diameter n homeomorphic to a complete graph K4.  Carlos Enrique Frasser, May 24 2018
a(n) + 4*a(n1) + a(n2) = n^3 = A000578(n), for n >= 0 (extending the a(n) formula given in the name). This is the Worpitzky identity for cubes. (Number of components of the decomposition of a rank 3 tensor in dimension n >= 1 into symmetric, mixed and antisymmetric parts). For a(n2) see my Dec 10 2015 comment.  Wolfdieter Lang, Jul 16 2019
a(n) also gives the total number of regular triangles of length k (in some length unit), with k from {1, 2, ..., n}, in the matchstick arrangement with enclosing triangle of length n, but only triangles with the orientation of the enclosing triangle are counted. Row sums of unsigned A122432(n1, k1), for n >= 1. See the Andrew Howroyd comment in A085691.  Wolfdieter Lang, Apr 06 2020
a(n) is the number of bigrassmannian permutations on n+1 elements, i.e., permutations which have a unique left descent, and a unique right descent.  Rafael Mrden, Aug 21 2020
a(n2) is the number of chiral pairs of colorings of the edges or vertices of a triangle using n or fewer colors.  Robert A. Russell, Oct 20 2020
a(n2) is the number of subsets of {1,2,...,n} whose diameters are their size. For example, for n=4, a(2)=4 and the sets are {1,3}, {2,4}, {1,2,4}, {1,3,4}.  Enrique Navarrete, Dec 26 2020
For n>1, a(n2) is the number of subsets of {1,2,...,n} in which the second largest element is the size of the subset. For example, for n=4, a(2)=4 and the sets are {2,3}, {2,4}, {1,3,4}, {2,3,4}.  Enrique Navarrete, Jan 02 2021
a(n) is the number of binary strings of length n+2 with exactly three 0's.  Enrique Navarrete, Jan 15 2021
Aside from the zero, this sequence is the fourth diagonal of the Pascal matrix A007318 and the only nonvanishing diagonal (fourth) of the matrix representation IM = (A132440)^3/3! of the differential operator D^3/3!, when acting on the row vector of coefficients of an o.g.f., or power series.
M = e^{IM} is the lower triangular matrix of coefficients of the Appell polynomial sequence p_n(x) = e^{D^3/3!} x^n = e^{b. D} x^n = (b. + x)^n = Sum_{k=0..n} binomial(n,k) b_n x^{nk}, where the (b.)^n = b_n have the e.g.f. e^{b.t} = e^{t^3/3!}, which is that for A025035 aerated with double zeros, the first column of M.
See A099174 and A000332 for analogous relationships for the third and fifth diagonals of the Pascal matrix. (End)
a(n) is the number of circles with a radius of integer length >= 1 and center at a grid point in an n X n grid.  Albert Swafford, Jun 11 2021
Maximum Wiener index over all connected graphs with n+1 vertices.  Allan Bickle, Jul 09 2022
The third Euler row (1,4,1) has an additional connection with the tetrahedral numbers besides the n^3 identity stated above: a^2(n) + 4*a^2(n+1) + a^2(n+2) = a(n^2+4n+4), which can be shown with algebra. E.g., a^2(2) + 4*a^2(3) + a^2(4) = 16 + 400 + 400 = a(16). Although an analogous thing happens with the (1,1) row of Euler's triangle and triangular numbers C(n+1,2) = A000217(n) = T(n), namely both T(n1) + T(n) = n^2 and T^2(n1) + T^2(n) = T(n^2) are true, only one (the usual identity) still holds for the Euler row (1,11,11,1) and the C(n,4) numbers in A000332. That is, the dot product of (1,11,11,1) with the squares of 4 consecutive terms of A000332 is not generally a term of A000332.  Richard Peterson, Aug 21 2022
For n > 1, a(n2) is the number of solutions of the Diophantine equation x1 + x2 + x3 + x4 + x5 = n, subject to the constraints 0 <= x1, 1 <= x2, 2 <= x3, 0 <= x4 <= 1, 0 <= x5 and x5 is even.  Daniel Checa, Nov 03 2022


REFERENCES

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 828.
V. I. Arnold (ed.), Arnold's Problems, Springer, 2004, comments on Problem 199011 (p. 75), pp. 503510. Numbers N_0.
A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 194.
J. H. Conway and R. K. Guy, The Book of Numbers, Copernicus Press, NY, 1996, p. 83.
H. S. M. Coxeter, Polyhedral numbers, pp. 2535 of R. S. Cohen, J. J. Stachel and M. W. Wartofsky, eds., For Dirk Struik: Scientific, historical and political essays in honor of Dirk J. Struik, Reidel, Dordrecht, 1974.
E. Deza and M. M. Deza, Figurate numbers, World Scientific Publishing (2012), page 93.
L. E. Dickson, History of the Theory of Numbers. Carnegie Institute Public. 256, Washington, DC, Vol. 1, 1919; Vol. 2, 1920; Vol. 3, 1923, see vol. 2, p. 4.
M. V. Diudea, I. Gutman, J. Lorentz, Molecular Topology, Nova Science, 2001, Huntington, N.Y. pp. 152156.
J. C. P. Miller, editor, Table of Binomial Coefficients. Royal Society Mathematical Tables, Vol. 3, Cambridge Univ. Press, 1954.
V. Ovsienko and S. Tabachnikov, Projective Differential Geometry Old and New, Cambridge Tracts in Mathematics (no. 165), Cambridge Univ. Press, 2005.
Kenneth A Ross, First Digits of Squares and Cubes, Math. Mag. 85 (2012) 3642. doi:10.4169/math.mag.85.1.36.
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).
A. Szenes, The combinatorics of the Verlinde formulas (N.J. Hitchin et al., ed.), in Vector bundles in algebraic geometry, Cambridge, 1995.
D. Wells, The Penguin Dictionary of Curious and Interesting Numbers, Penguin Books, 1987, pp. 126127.
B. Zwiebach, A First Course in String Theory, Cambridge, 2004; see p. 226.


LINKS

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
T. P. Martin, Shells of atoms, Phys. Reports, 273 (1996), 199241, eq. (1).


FORMULA

a(n) = C(n+2,3) = n*(n+1)*(n+2)/6 (see the name).
G.f.: x / (1  x)^4.
a(n) = a(4  n) for all in Z.
a(n) = Sum_{k=0..n} A000217(k) = Sum_{k=1..n} Sum_{j=0..k} j, partial sums of the triangular numbers.
Determinant of the n X n symmetric Pascal matrix M_(i, j) = C(i+j+2, i).  Benoit Cloitre, Aug 19 2003
The sum of a series constructed by the products of the index and the length of the series (n) minus the index (i): a(n) = sum[i(ni)].  Martin Steven McCormick (mathseq(AT)wazer.net), Apr 06 2005
a(n) = Sum_{k=0..floor((n1)/2)} (n2k)^2 [offset 0]; a(n+1) = Sum_{k=0..n} k^2*(1(1)^(n+k1))/2 [offset 0].  Paul Barry, Apr 16 2005
Values of the Verlinde formula for SL_2, with g = 2: a(n) = Sum_{j=1..n1} n/(2*sin^2(j*Pi/n)).  Simone Severini, Sep 25 2006
a(n) = Sum_{k=1..n} binomial(n*k+1, n*k1), with a(0) = 0.  Paolo P. Lava, Apr 13 2007
a(n1) = (1/(1!*2!))*Sum_{1 <= x_1, x_2 <= n} det V(x_1, x_2) = (1/2)*Sum_{1 <= i,j <= n} ij, where V(x_1, x_2) is the Vandermonde matrix of order 2. Column 2 of A133112.  Peter Bala, Sep 13 2007
Starting with 1 = binomial transform of [1, 3, 3, 1, ...]; e.g., a(4) = 20 = (1, 3, 3, 1) dot (1, 3, 3, 1) = (1 + 9 + 9 + 1).  Gary W. Adamson, Nov 04 2007
Sum_{n>=1} 1/a(n) = 3/2, case x = 1 in GradsteinRyshik 1.513.7.  R. J. Mathar, Jan 27 2009
With offset 1, a(n) = (1/6)*floor(n^5/(n^2 + 1)).  Gary Detlefs, Feb 14 2010
a(n) = (3*n^2 + 6*n + 2)/(6*(h(n+2)  h(n1))), n > 0, where h(n) is the nth harmonic number.  Gary Detlefs, Jul 01 2011
a(n) = coefficient of x^2 in the Maclaurin expansion of 1 + 1/(x+1) + 1/(x+1)^2 + 1/(x+1)^3 + ... + 1/(x+1)^n.  Francesco Daddi, Aug 02 2011
a(n) = coefficient of x^4 in the Maclaurin expansion of sin(x)*exp((n+1)*x).  Francesco Daddi, Aug 04 2011
a(n) = 3*a(n1)  3*a(n2) + a(n3) + 1.  Ant King, Oct 18 2012
G.f.: x*U(0) where U(k) = 1 + 2*x*(k+2)/( 2*k+1  x*(2*k+1)*(2*k+5)/(x*(2*k+5)+(2*k+2)/U(k+1) )); (continued fraction, 3rd kind, 3step).  Sergei N. Gladkovskii, Dec 01 2012
a(n^2  1) = (1/2)*(a(n^2  n  2) + a(n^2 + n  2)) and
G.f.: x + 4*x^2/(Q(0)4*x) where Q(k) = 1 + k*(x+1) + 4*x  x*(k+1)*(k+5)/Q(k+1); (continued fraction).  Sergei N. Gladkovskii, Mar 14 2013
a(n+1) = det(C(i+3,j+2), 1 <= i,j <= n), where C(n,k) are binomial coefficients.  Mircea Merca, Apr 06 2013
G.f.: x*G(0)/2, where G(k) = 1 + 1/(1  x/(x + (k+1)/(k+4)/G(k+1))); (continued fraction).  Sergei N. Gladkovskii, Jun 02 2013
a(n)*(m+1)^3 + a(m)*(n+1) = a(n*m + n + m), for any nonnegative integers m and n. This is a 3D analog of Euler's theorem about triangular numbers, namely t(n)*(2m+1)^2 + t(m) = t(2nm + n + m), where t(n) is the nth triangular number.  Ivan N. Ianakiev, Aug 20 2013
Sum_{n>=0} a(n)/(n+1)! = 2*e/3 = 1.8121878856393... . Sum_{n>=1} a(n)/n! = 13*e/6 = 5.88961062832... .  Richard R. Forberg, Dec 25 2013
a(k*n) = a(k)*a(n) + 4*a(k1)*a(n1) + a(k2)*a(n2).  Robert Israel, Apr 20 2015
Dirichlet g.f.: (zeta(s3) + 3*zeta(s2) + 2*zeta(s1))/6.  Ilya Gutkovskiy, Jul 01 2016
G.f.: x/(1  x)^4 = (x * r(x) * r(x^2) * r(x^4) * r(x^8) * ...), where r(x) = (1 + x)^4 = (1 + 4x + 6x^2 + 4x^3 + x^4); and x/(1  x)^4 = (x * r(x) * r(x^3) * r(x^9) * r(x^27) * ...) where r(x) = (1 + x + x^2)^4.  Gary W. Adamson, Jan 23 2017
a(n) = 1*C(n,1) + 2*C(n,2) + 1*C(n,3), where the coefficient of C(n,k) is the number of unoriented triangle colorings using exactly k colors.
a(n2) = 1*C(n,3), where the coefficient of C(n,k) is the number of chiral pairs of triangle colorings using exactly k colors.
Product_{n>=1} (1 + 1/a(n)) = sinh(sqrt(2)*Pi)/(3*sqrt(2)*Pi).
Product_{n>=2} (1  1/a(n)) = sqrt(2)*sinh(sqrt(2)*Pi)/(33*Pi). (End)


EXAMPLE

a(2) = 3*4*5/6 = 10, the number of balls in a pyramid of 3 layers of balls, 6 in a triangle at the bottom, 3 in the middle layer and 1 on top.
Consider the square array
1 2 3 4 5 6 ...
2 4 6 8 10 12 ...
3 6 9 12 16 20 ...
4 8 12 16 20 24 ...
5 10 15 20 25 30 ...
...
G.f. = x + 4*x^2 + 10*x^3 + 20*x^4 + 35*x^5 + 56*x^6 + 84*x^7 + 120*x^8 + 165*x^9 + ...
Example for a(3+1) = 20 nondecreasing 3letter words over {1,2,3,4}: 111, 222, 333; 444, 112, 113, 114, 223, 224, 122, 224, 133, 233, 144, 244, 344; 123, 124, 134, 234. 4 + 4*3 + 4 = 20.  Wolfdieter Lang, Jul 29 2014
Example for a(42) = 4 independent components of a rank 3 antisymmetric tensor A of dimension 4: A(1,2,3), A(1,2,4), A(1,3,4) and A(2,3,4).  Wolfdieter Lang, Dec 10 2015


MAPLE

a:=n>n*(n+1)*(n+2)/6; seq(a(n), n=0..50);


MATHEMATICA

FoldList[Plus, 0, Rest[FoldLtetrist[Plus, 0, Range[50]]]] (* corrected by Robert G. Wilson v, Jun 26 2013 *)
Accumulate[Accumulate[Range[0, 50]]] (* Harvey P. Dale, Dec 10 2011 *)
LinearRecurrence[{4, 6, 4, 1}, {0, 1, 4, 10}, 20] (* Eric W. Weisstein, Sep 08 2017 *)
CoefficientList[Series[x/(1 + x)^4, {x, 0, 20}], x] (* Eric W. Weisstein, Sep 08 2017 *)


PROG

(PARI) a(n) = (n) * (n+1) * (n+2) / 6 \\ corrected by Harry J. Smith, Dec 22 2008
(PARI) a=vector(10000); a[2]=1; for(i=3, #a, a[i]=a[i2]+i*i); \\ Stanislav Sykora, Nov 07 2013
(DERIVE) v(n):= [1, 2, 3, ..., n] w(n):= [n, ..., 3, 2, 1] a(n):= scalar product (v(n)w(n)) " From Roland Schroeder (florola(AT)gmx.de), Aug 14 2010 "
(Haskell)
a000292 n = n * (n + 1) * (n + 2) `div` 6
a000292_list = scanl1 (+) a000217_list
x, y, z = 1, 1, 1
yield 0
while True:
yield x
x, y, z = x + y + z + 1, y + z + 1, z + 1


CROSSREFS

Sums of 2 consecutive terms give A000330.
Cf. A000217 (first differences), A001044, (see above example), A061552, A040977, A133111, A133112, A152205, A158823, A156925, A157703, A173564, A058187, A190717, A190718, A100440, A181118, A222716.
Cf. A216499 (the analogous sequence for level1 phylogenetic networks).
Cf. similar sequences listed in A237616.
Cf. A104712 (second column, if offset is 2).
Cf. A002817 (4cycle count of \bar P_{n+4}), A060446 (5cycle count of \bar P_{n+3}), A302695 (6cycle count of \bar P_{n+5})
Row 2 of A325000 (simplex facets and vertices) and A327084 (simplex edges and ridges).
Cf. (triangle colorings) A006527 (oriented), A000290 (achiral), A327085 (chiral simplex edges and ridges).
Row 3 of A321791 (cycles of n colors using k or fewer colors).


KEYWORD

nonn,core,easy,nice


AUTHOR



EXTENSIONS



STATUS

approved



