Demonstration of the
OnLine Encyclopedia of Integer Sequences® (OEIS®)
(Page 3)
Identifying a Sequence: Supplying a Formula
A secondary goal of the
OEIS
is to provide a place where the general public has access to interesting
parts of mathematics.
Suppose someone rediscovers the sequence of tetrahedral
numbers, the number of balls in a triangular pyramid,
shown here:
The first few numbers are easy to calculate by hand:
1, 4, 10, 20, 35, 56, ...
This person might be a highschool student in Tokyo,
a medical doctor in Paris,
or a retired mountain climber in South Dakota.
He or she would like to know if
there is a formula for these numbers, what they are called,
and a reference where they can find out more about them.
As long as they have access to the Internet or to electronic mail,
they can consult the
OEIS.
(If they don't have access to either the Internet or email,
even if they do not have electricity  like the correspondent in South Dakota 
they can still refer to the
book version,
published in 1995 by Academic Press. This is now out of date,
but includes some 5000 of the most important sequences.)
For the moment, let us suppose they can access the Internet.
(Consulting the database via email will be discussed in a later demonstration.)
They go to the
main web page,
where they see the following.
You replace the example by your sequence and click "Submit":
The reply shows several sequences that match these terms, but the top
entry is the sequence that is sought:


A000292


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


326
 

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
(list; graph; refs; listen; history; edit; internal format)



OFFSET
 0,3


COMMENTS
 a(n) = number of balls in a triangular pyramid in which each edge contains n+1 balls. The sum of the first n triangular numbers (A000217).
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 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 (helms(AT)unikassel.de), Apr 15 2002
a(n) = sum ij for all 1 <= i <= j <= n.  Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Aug 05 2002
a(n) = sum of the all possible products p*q where (p,q) are ordered pairs and p+q = n+1. a(5) = 5 + 8 + 9 + 8 + 5 = 35.  Amarnath Murthy (amarnath_murthy(AT)yahoo.com), May 29 2003
Number of labeled graphs on n+3 nodes that are triangles.  Jon Perry (perry(AT)globalnet.co.uk), Jun 14 2003
Number of permutations of n+3 which have exactly 1 descent and avoid the pattern 1324.  Mike Zabrocki (zabrocki(AT)mathstat.yorku.ca), 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) = A108299(n+5,6) = A108299(n+6,7).  Reinhard Zumkeller, Jun 01 2005
a(n) = A110555(n+4,3).  Reinhard Zumkeller, Jul 27 2005
a(n) is a perfect square only for n = {1, 2, 48}. a(48) = 19600 = 140^2.  Alexander Adamchuk (alex(AT)kolmogorov.com), 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 (sfalcon(AT)dma.ulpgc.es), Feb 12 2007. (Corrected by Graeme McRae (g_m(AT)mcraefamily.com), Aug 28 2007)
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)=diff(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 R. Janjic (agnus(AT)blic.net), Aug 15 2007
Complement of A145397; A023533(a(n))=1; A014306(a(n))=0. [From Reinhard Zumkeller, Oct 14 2008]
Equals row sums of triangle A152205 [From Gary W. Adamson, Nov 29 2008]
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. [From Bernard Hill (bernard(AT)braeburn.co.uk), Dec 05 2008]
From Johannes W. Meijer, Mar 07 2009: (Start)
Sequence of the absolute values of the z^1 coefficients of the polynomials in the GF2 denominators of A156925. See A157703 for background information.
(End)
Starting with 1 = row sums of triangle A158823 [From Gary W. Adamson, Mar 28 2009]
Wiener index of the path graph P_n [From Eric W. Weisstein, Apr 30 2009]
From Peter Luschny, Jul 14 2009: (Start)
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); (End)
a(n) is the number of nondecreasing, threeelement permutations of n distinct numbers. [From Samuel Savitz, Sep 12 2009]
a(n+4) = Number of different partitions of number n on sum of 4 elements a(6)=a(2+4)becuse we have 10 different partionions 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 [From Artur Jasinski (grafix(AT)csl.pl), Nov 30 2009]
a(n) corresponds to the total number of steps to memorize n verses by the technique described in A173564. [From Ibrahima Faye (ifaye2001(AT)yahoo.fr), Feb 22 2010]
a(n) is also given by a very small DERIVEprogram: v(n) := VECTOR(k, k, 1, n) w(n) := VECTOR(n  k, k, 0, n  1) a(n) := v(n) [nonascii characters here] cents w(n) [From Roland Schroeder (florola(AT)gmx.de), Jul 12 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 ngones by intersecting diagonals with three diagonal endpoints. Ref.: Steven E. Sommers in: Journ. of Integer Sequences, Vol. 1 (1998), Article 98.1.5 (see the first column of the table): http://www.cs.uwaterloo.ca/journals/JIS/sommars/newtriangle.html [Alexandre Wajnberg (alexandre.wajnberg(AT)skynet.be), Aug 21 2010.
Column sums of:
1 4 9 16 25...
1 4 9...
1...
..............

1 4 10 20 35...
From Johannes W. Meijer, May 20 2011: (Start)
The Ca3, Ca4, Gi3 and Gi4 triangle sums, for their definitions see A180662, 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. [From 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 differerences 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,    . [Kailasam Viswanathan Iyer, July 8 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


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.
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.
T. P. Martin, Shells of atoms, Phys. Reports, 273 (1996), 199241, eq. (1).
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.
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, pp. 1267 Penguin Books 1987.
B. Zwiebach, A First Course in String Theory, Cambridge, 2004; see p. 226


LINKS
 N. J. A. Sloane, Table of n, a(n) for n = 0..10000
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].
O. Aichholzer and H. Krasser, The point set order type data base: a collection of applications and results, pp. 1720 in Abstracts 13th Canadian Conference on Computational Geometry (CCCG '01), Waterloo, Aug. 1315, 2001.
P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.
N. Heninger, E. M. Rains and N. J. A. Sloane, On the Integrality of nth Roots of Generating Functions, J. Combinatorial Theory, Series A, 113 (2006), 17321745.
Milan Janjic, Two Enumerative Functions
R. Jovanovic, First 2500 Tetrahedral numbers
Hyun Kwang Kim, On Regular Polytope Numbers
Alexsandar Petojevic, The Function vM_m(s; a; z) and Some WellKnown Sequences, Journal of Integer Sequences, Vol. 5 (2002), Article 02.1.7
N. J. A. Sloane, Illustration of initial terms
N. J. A. Sloane, Pyramid of 20 balls corresponding to a(3)=20.
G. Villemin's Almanach of Numbers, Nombres Tetraedriques
Eric Weisstein's World of Mathematics, Tetrahedral Number, Composition, Wiener Index
Index entries for "core" sequences
Index entries for twoway infinite sequences
Index entries for sequences related to linear recurrences with constant coefficients, signature (4,6,4,1)


FORMULA
 G.f.: x/(1x)^4.
a(n)=4*a(n1)6*a(n2)+4*a(n3)a(n4) for n>=4. [Jaume Oliver Lafont, Nov 18 2008]
a(4n)=a(n).
E.g.f.:((x^3)/6+x^2+x)*exp(x) [From Geoffrey Critzer, Feb 21 2009]
a(n) = sum_{k=1..n} k*(nk+1). [Vladimir Shevelev, Jul 30 2010]
Partial sums of the triangular numbers (A000217).
a(n) = (n+3)*a(n1)/n.  Ralf Stephan, Apr 26 2003
Sums of three consecutive terms give A006003.  Ralf Stephan, Apr 26 2003
a(n) = C(1,2)+C(2,2)+...+C(n1,2)+C(n,2); e.g. for n=5: a(5)=0+1+3+6+10=20.  Labos E. (labos(AT)ana.sote.hu), May 09 2003
Determinant of the n X n symmetric Pascal matrix M_(i, j)=C(i+j+2, i)  Benoit Cloitre (benoit7848c(AT)orange.fr), 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)]. Also the sum of n terms of A000217.  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[ Sum[ k, {k,1,m} ], {m,1,n} ].  Alexander Adamchuk, Oct 28 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
a(n) = A006503(n)  A002378(n). [From Reinhard Zumkeller, Sep 24 2008]
Sum_{n=1..infinity} 1/a(n) = 3/2, case x=1 in GradsteinRyshik 1.513.7. [From R. J. Mathar, Jan 27 2009]
Lim{n>oo} A171973(n)/a(n) = sqrt(2)/2. [From Reinhard Zumkeller, Jan 20 2010]
With offset 1, a(n) =(1/6)*floor(n^5/(n^2+1)) [From 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. [From 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. [From Francesco Daddi, Aug 02 2011]
a(n)=coefficient of x^4 in the Maclaurin expansion of sin(x)*exp((n+1)*x). [From Francesco Daddi, Aug 04 2011]
a(n)= 2*A002415(n+1)/(n+1)  Tom Copeland, Sep 13 2011


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...
...
then a(n) = sum of nth antidiagonal.  Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Apr 06 2003


MAPLE
 a:=n>n*(n+1)*(n+2)/6;
A000292 := n>binomial(n+3, 3);


MATHEMATICA
 Table[Binomial[n + 3, 3], {n, 0, 20}] [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jan 31 2010]
Rest[FoldList[Plus, 0, Rest[FoldList[Plus, 0, Range[50]]]]]


PROG
 (PARI) a(n)=(n)*(n+1)*(n+2)/6
(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]


CROSSREFS
 Bisections give A000447 and A002492.
Sums of 2 consecutive terms give A000330.
a(3n3)=A006566(n). A000447(n)=a(2n2). A002492(n)=a(2n+1).
First differences give triangular numbers.
Column 0 of triangle A094415.
Cf. A000217, A001044, A003991, A061552.
Cf. A040977, A133111, A133112.
Cf. A152205 [From Gary W. Adamson, Nov 29 2008]
Cf. A156925, A157703.
Cf. A158823 [From Gary W. Adamson, Mar 28 2009]
Cf. A173564 [From Ibrahima Faye (ifaye2001(AT)yahoo.fr), Feb 22 2010]
Partial sums are A000332. [From Jonathan Vos Post, Mar 27 2011]
Cf. A058187, A190717, A190718. [From Johannes W. Meijer, May 20 2011]
Sequence in context: A138778 A038409 A090579 * A101552 A038419 A057319
Adjacent sequences: A000289 A000290 A000291 * A000293 A000294 A000295


KEYWORD
 nonn,core,easy,nice


AUTHOR
 N. J. A. Sloane (njas(AT)research.att.com).


EXTENSIONS
 More terms from Michael Somos
Corrected PARI program.  Harry J. Smith (hjsmithh(AT)sbcglobal.net), Dec 22 2008
Multiplied g.f. with x to match the offset R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Apr 23 2009
Corrected and edited by Daniel Forgues (squid(AT)zensearch.com), May 14 2010



The reply gives more terms, the name of the sequence,
a formula for the nth term, a generating function,
and several references and links where they can find out more
about the sequence.
The Beiler reference in particular (a wonderful book) has lured many people into
studying mathematics for pleasure.
No doubt the new book by Conway and Guy (also highly recommended for general
readers) will accomplish the same thing.
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