

A002620


Quartersquares: floor(n/2)*ceiling(n/2). Equivalently, floor(n^2/4).
(Formerly M0998 N0374)


320



0, 0, 1, 2, 4, 6, 9, 12, 16, 20, 25, 30, 36, 42, 49, 56, 64, 72, 81, 90, 100, 110, 121, 132, 144, 156, 169, 182, 196, 210, 225, 240, 256, 272, 289, 306, 324, 342, 361, 380, 400, 420, 441, 462, 484, 506, 529, 552, 576, 600, 625, 650, 676, 702, 729, 756, 784, 812
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OFFSET

0,4


COMMENTS

b(n) = A002620(n+2) = number of multigraphs with loops on 2 nodes with n edges [so g.f. for b(n) is 1/((1x)^2*(1x^2))]. Also number of 2covers of an nset; also number of 2 X n binary matrices with no zero columns up to row and column permutation.  Vladeta Jovovic, Jun 08 2000
a(n) is also the maximal number of edges that a trianglefree graph of n vertices can have. For n = 2m, the maximum is achieved by the bipartite graph K(m, m); for n = 2m + 1, the maximum is achieved by the bipartite graph K(m, m + 1).  Avi Peretz (njk(AT)netvision.net.il), Mar 18 2001
a(n) is the number of arithmetic progressions of 3 terms and any mean which can be extracted from the set of the first n natural numbers (starting from 1).  Santi Spadaro, Jul 13 2001
This is also the order dimension of the (strong) Bruhat order on the Coxeter group A_{n1} (the symmetric group S_n).  Nathan Reading (reading(AT)math.umn.edu), Mar 07 2002
Let M_n denote the n X n matrix m(i,j) = 2 if i = j; m(i, j) = 1 if (i+j) is even; m(i, j) = 0 if i + j is odd, then a(n+2) = det M_n.  Benoit Cloitre, Jun 19 2002
Sums of pairs of neighboring terms are triangular numbers in increasing order.  Amarnath Murthy, Aug 19 2002
Also, from the starting position in standard chess, minimum number of captures by pawns of the same color to place n of them on the same file (column). Beyond a(6), the board and number of pieces available for capture are assumed to be extended enough to accomplish this task.  Rick L. Shepherd, Sep 17 2002
For example, a(2) = 1 and one capture can produce "doubled pawns", a(3) = 2 and two captures is sufficient to produce tripled pawns, etc. (Of course other, uncounted, noncapturing pawn moves are also necessary from the starting position in order to put three or more pawns on a given file.)  Rick L. Shepherd, Sep 17 2002
Terms are the geometric mean and arithmetic mean of their neighbors alternately.  Amarnath Murthy, Oct 17 2002
Maximum product of two integers whose sum is n.  Matthew Vandermast, Mar 04 2003
a(n+1) gives number of nonsymmetric partitions of n into at most 3 parts, with zeros used as padding. E.g., a(6) = 12 because we can write 5 = 5 + 0 + 0 = 0 + 5 + 0 = 4 + 1 + 0 = 1 + 4 + 0 = 1 + 0 + 4 = 3 + 2 + 0 = 2 + 3 + 0 = 2 + 0 + 3 = 2 + 2 + 1 = 2 + 1 + 2 = 3 + 1 + 1 = 1 + 3 + 1.  Jon Perry, Jul 08 2003
a(n1) gives number of distinct elements greater than 1 of nonsymmetric partitions of n into at most 3 parts, with zeros used as padding, appear in the middle. E.g., 5 = 5 + 0 + 0 = 0 + 5 + 0 = 4 + 1 + 0 = 1 + 4 + 0 = 1 + 0 + 4 = 3 + 2 + 0 = 2 + 3 + 0 = 2 + 0 + 3 = 2 + 2 + 1 = 2 + 1 + 2 = 3 + 1 + 1 = 1 + 3 + 1. Of these, 050, 140, 320, 230, 221, 131 qualify and a(4) = 6.  Jon Perry, Jul 08 2003
Union of square numbers (A000290) and oblong numbers (A002378).  Lekraj Beedassy, Oct 02 2003
Conjectured size of the smallest critical set in a Latin square of order n (true for n <= 8).  Richard Bean (rwb(AT)eskimo.com), Jun 12 2003 and Nov 18 2003
a(n) gives number of maximal strokes on complete graph K_n, when edges on K_n can be assigned directions in any way. A "stroke" is a locally maximal directed path on a directed graph. Examples: n = 3, two strokes can exist, "x > y > z" and " x > z", so a(3) = 2 . n = 4, four maximal strokes exist, "u > x > z" and "u > y" and "u > z" and "x > y > z", so a(4) = 4.  Yasutoshi Kohmoto, Dec 20 2003
Number of symmetric Dyck paths of semilength n+1 and having three peaks. E.g., a(4) = 4 because we have U*DUUU*DDDU*D, UU*DUU*DDU*DD, UU*DDU*DUU*DD and UUU*DU*DU*DDD, where U = (1, 1), D = (1, 1) and * indicates a peak.  Emeric Deutsch, Jan 12 2004
Number of valid inequalities of the form j + k < n + 1, where j and k are positive integers, j <= k, n >= 0.  Rick L. Shepherd, Feb 27 2004
See A092186 for another application.
Also, the number of nonisomorphic transversal combinatorial geometries of rank 2.  Alexandr S. Radionov (rasmailru(AT)mail.ru), Jun 02 2004
a(n+1) is the transform of n under the Riordan array (1/(1x^2), x).  Paul Barry, Apr 16 2005
1, 2, 4, 6, 9, 12, 16, 20, 25, 30, ... specifies the largest number of copies of any of the gifts you receive on the nth day in the "Twelve Days of Christmas" song. For example, on the fifth day of Christmas, you have 9 French hens.  Alonso del Arte, Jun 17 2005
a(n) = Sum_{k=0..n} Min{k, nk}, sums of rows of the triangle in A004197.  Reinhard Zumkeller, Jul 27 2005
a(n+1) is the number of noncongruent integersided triangles with largest side n.  David W. Wilson [Comment corrected Sep 26 2006]
A quartersquare table can be used to multiply integers since n*m = a(n+m)  a(nm) for all integer n, m.  Michael Somos, Oct 29 2006
The sequence is the size of the smallest strong critical set in a Latin square of order n.  G.H.J. van Rees (vanrees(AT)cs.umanitoba.ca), Feb 16 2007
Maximal number of squares (maximal area) in a polyomino with perimeter 2n.  Tanya Khovanova, Jul 04 2007
For n >= 3 a(n1) is the number of bracelets with n+3 beads, 2 of which are red, 1 of which is blue.  Washington Bomfim, Jul 26 2008
Equals row sums of triangle A122196.  Gary W. Adamson, Nov 29 2008
Also a(n) is the number of different patterns of a 2colored 3partition of n.  Ctibor O. Zizka, Nov 19 2014
Also a(n1) = C(((n+(n mod 2))/2), 2) + C(((n(n mod 2))/2), 2), so this is the second diagonal of A061857 and A061866, and each evenindexed term is the average of its two neighbors.  Antti Karttunen
Equals triangle A171608 * ( 1, 2, 3, ...).  Gary W. Adamson, Dec 12 2009
a(n) gives the number of nonisomorphic faithful representations of the Symmetric group S_3 of dimension n. Any faithful representation of S_3 must contain at least one copy of the 2dimensional irrep, along with any combination of the two 1dimensional irreps.  Andrew Rupinski, Jan 20 2011
a(n+2) gives the number of ways to make change for "c" cents, letting n = floor(c/5) to account for the 5repetitive nature of the task, using only pennies, nickels and dimes (see A187243).  Adam Sasson, Mar 07 2011
a(n) belongs to the sequence if and only if a(n) = floor(sqrt(a(n))) * ceiling(sqrt(a(n))), that is, a(n) = k^2 or a(n) = k*(k+1), k >= 0.  Daniel Forgues, Apr 17 2011
a(n) is the sum of the positive integers < n that have the opposite parity as n.
Deleting the first 0 from the sequence results in a sequence b = 0, 1, 2, 4, ... such that b(n) is sum of the positive integers <= n that have the same parity as n. The sequence b(n) is the additive counterpart of the double factorial.  Peter Luschny, Jul 06 2011
Third outer diagonal of Losanitsch's Triangle, A034851.  Fred Daniel Kline, Sep 10 2011
Written as a(1) = 1, a(n) = a(n1) + ceiling (a(n1)) this is to ceiling as A002984 is to floor, and as A033638 is to round.  Jonathan Vos Post, Oct 08 2011
a(n2) gives the number of distinct graphs with n vertices and n regions.  Erik Hasse, Oct 18 2011
Construct the nth row of Pascal's triangle (A007318) from the preceding row, starting with row 0 = 1. a(n) counts the total number of additions required to compute the triangle in this way up to row n, with the restrictions that copying a term does not count as an addition, and that all additions not required by the symmetry of Pascal's triangle are replaced by copying terms.  Douglas Latimer, Mar 05 2012
a(n) is the sum of the positive differences of the parts in the partitions of n+1 into exactly 2 parts.  Wesley Ivan Hurt, Jan 27 2013
a(n) is the maximum number of covering relations possible in an nelement graded poset. For n = 2m, this bound is achieved for the poset with two sets of m elements, with each point in the "upper" set covering each point in the "lower" set. For n = 2m+1, this bound is achieved by the poset with m nodes in an upper set covering each of m+1 nodes in a lower set.  Ben Branman, Mar 26 2013
a(n+2) is the number of (integer) partitions of n into 2 sorts of 1's and 1 sort of 2's.  Joerg Arndt, May 17 2013
Alternative statement of Oppermann's conjecture: For n>2, there is at least one prime between a(n) and a(n+1).  Ivan N. Ianakiev, May 23 2013. [This conjecture was mentioned in A220492, A222030.  Omar E. Pol, Oct 25 2013]
For any given prime number, p, there are an infinite number of a(n) divisible by p, with those a(n) occurring in evenly spaced clusters of three as a(n), a(n+1), a(n+2) for a given p. The divisibility of all a(n) by p and the result are given by the following equations, where m >= 1 is the cluster number for that p: a(2m*p)/p = p*m^2  m; a(2m*p + 1)/p = p*m^2; a(2m*p + 2)/p = p*m^2 + m. The number of a(n) instances between clusters is 2*p  3.  Richard R. Forberg, Jun 09 2013
Apart from the initial term this is the elliptic troublemaker sequence R_n(1,2) in the notation of Stange (see Table 1, p.16). For other elliptic troublemaker sequences R_n(a,b) see the cross references below.  Peter Bala, Aug 08 2013
a(n) is also the total number of twin hearts patterns (6c4c) packing into (n+1) X (n+1) coins, the coins left is A042948 and the voids left is A000982. See illustration in links.  Kival Ngaokrajang, Oct 24 2013
Partitions of 2n into parts of size 1, 2 or 4 where the largest part is 4, i.e., A073463(n,2).  Henry Bottomley, Oct 28 2013
a(n+1) is the minimum length of a sequence (of not necessarily distinct terms) that guarantees the existence of a (not necessarily consecutive) subsequence of length n in which like terms appear consecutively. This is also the minimum cardinality of an ordered set S that ensures that, given any partition of S, there will be a subset T of S so that the induced subpartition on T avoids the pattern ac/b, where a < b < c.  Eric Gottlieb, Mar 05 2014
A237347(a(n)) = 3; A235711(n) = A003415(a(n)).  Reinhard Zumkeller, Mar 18 2014
Also the number of elements of the list 1..n+1 such that for any two elements {x,y} the integer (x+y)/2 lies in the range ]x,y[.  Robert G. Wilson v, May 22 2014
Number of lattice points (x,y) inside the region of the coordinate plane bounded by x<=n, 0<y<=x/2. For a(11)=30 there are exactly 30 lattice points in the region below:
6 .
. . 
5 .__+__+
. .   
4 .__+__+__+__+
. .     
3 .__+__+__+__+__+__+
. .       
2 .__+__+__+__+__+__+__+__+
. .         
1 .__+__+__+__+__+__+__+__+__+__+
..           
0.__+__+__+__+__+__+__+__+__+__+__+_________
0 1 2 3 4 5 6 7 8 9 10 11 .. n
0 0 1 2 4 6 9 12 16 20 25 30 .. a(n)  Wesley Ivan Hurt, Oct 26 2014
a(n+1) is the greatest integer k for which there exists an n x n matrix M of nonnegative integers with every row and column summing to k, such that there do not exist n entries of M, all greater than 1, and no two of these entries in the same row or column.  Richard Stanley, Nov 19 2014
In a tiling of the triangular shape T_N with row length k for row k = 1, 2, ..., N >=1 (or, alternatively row length N = 1k for row k) with rectangular tiles, there can appear rectangles (i, j), N >= i >= j >= 1, of a(N+1) types (and their transposed shapes obtained by interchanging i and j). See the Feb 27 2004 comment above from Rick L. Shepherd. The motivation to look into this came from a proposal of Kival Ngaokrajang in A247139.  Wolfdieter Lang, Dec 09 2014
Every positive integer is a sum of at most four distinct quartersquares; see A257018.  Clark Kimberling, Apr 15 2015
a(n+1) gives the maximal number of distinct elements of an n X n matrix which is symmetric (w.r.t. the main diagonal) and symmetric w.r.t. the main antidiagonal. Such matrices are called bisymmetric. See the Wikipedia link.  Wolfdieter Lang, Jul 07 2015
For 2^a(n+1), n >= 1, the number of binary bisymmetric n X n matrices, see A060656(n+1) and the comment and link by Dennis P. Walsh.  Wolfdieter Lang, Aug 16 2015
a(n) is the number of partitions of 2n+1 of length three with exactly two even entries (see below example).  John M. Campbell, Jan 29 2016
a(n) is the sum of the asymmetry degrees of all 01avoiding binary words of length n. The asymmetry degree of a finite sequence of numbers is defined to be the number of pairs of symmetrically positioned distinct entries. a(6) = 9 because the 01avoiding binary words of length 6 are 000000, 100000, 110000, 111000, 111100, 111110, and 111111, and the sum of their asymmetry degrees is 0 + 1 + 2 + 3 + 2 + 1 + 0 = 9. Equivalently, a(n) = Sum(k*A275437(n,k), k>=0).  Emeric Deutsch, Aug 15 2016
a(n) is the number of ways to represent all the integers in the interval [3,n+1] as the sum of two distinct natural numbers. E.g., a(7)=12 as there are 12 different ways to represent all the numbers in the interval [3,8] as the sum of two distinct parts: 1+2=3, 1+3=4, 1+4=5, 1+5=6, 1+6=7, 1+7=8, 2+3=5, 2+4=6, 2+5=7, 2+6=8, 3+4=7, 3+5=8.  Anton Zakharov, Aug 24 2016
a(n+2) is the number of conjugacy classes of involutions (considering the identity as an involution) in the hyperoctahedral group C_2 wreath S_n.  Mark Wildon, Apr 22 2017


REFERENCES

Sergei Abramovich, Combinatorics of the Triangle Inequality: From Straws to Experimental Mathematics for Teachers, Spreadsheets in Education (eJSiE), Vol. 9, Is
sue 1, Article 1, 2016. See Fig. 3.
G. L. Alexanderson et al., The William Powell Putnam Mathematical Competition  Problems and Solutions: 19651984, M.A.A., 1985; see Problem A1 of 27th Competition.
T. M. Apostol, Introduction to Analytic Number Theory, SpringerVerlag, 1976, page 73, problem 25.
R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics. AddisonWesley, Reading, MA, 1990, p. 99.
D. E. Knuth, The art of programming, Vol. 1, 3rd Edition, AddisonWesley, 1997, Ex. 36 of section 1.2.4.
J. Nelder, Critical sets in Latin squares, CSIRO Division of Math. and Stats. Newsletter, Vol. 38 (1977), p. 4.
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).


LINKS

Franklin T. AdamsWatters, Table of n, a(n) for n = 0..10000
J. A. Bate & G. H. J. van Rees, The Size of the Smallest Strong Critical Set in a Latin Square, Ars Combinatoria, Vol. 53 (1999) 7383.
M. Benoumhani, M. Kolli, Finite topologies and partitions, JIS 13 (2010) # 10.3.5, Lemma 6 first line.
G. Blom and C.E. Froeberg, Om myntvaexling (On moneychanging) [Swedish], Nordisk Matematisk Tidskrift, 10 (1962), 5569, 103. [Annotated scanned copy] See Table 4, row 3.
Washington G. Bomfim, Illustration of the bracelets with 8 beads, 2 of which are red, 1 of which is blue..
H. Bottomley, Illustration of initial terms
J. Brandts and C. Cihangir, Counting triangles that share their vertices with the unit ncube, in Conference Applications of Mathematics 2013 in honor of the 70th birthday of Karel Segeth. Jan Brandts, Sergey Korotov, et al., eds., Institute of Mathematics AS CR, Prague 2013.
Jan Brandts, A Cihangir, Enumeration and investigation of acute 0/1simplices modulo the action of the hyperoctahedral group, arXiv preprint arXiv:1512.03044 [math.CO], 2015.
P. J. Cameron, BCC Problem List, Problem BCC15.15 (DM285), Discrete Math. 167/168 (1997), 605615.
P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.
E. Fix and J. L. Hodges, Jr., Significance probabilities of the Wilcoxon test, Annals Math. Stat., 26 (1955), 301312.
E. Fix and J. L. Hodges, Significance probabilities of the Wilcoxon test, Annals Math. Stat., 26 (1955), 301312. [Annotated scanned copy]
A. Ganesan, Automorphism groups of graphs, arXiv preprint arXiv:1206.6279 [cs.DM], 2012.  From N. J. A. Sloane, Dec 17 2012
E. Gottlieb, M. Sheard, An ErdosSzekeres result for set partitions, Slides from a talk, Nov 14 2014. [A006260 is a typo for A002620]
R. K. Guy, Letters to N. J. A. Sloane, JuneAugust 1968
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 105
O. A. Ivanov, On the number of regions into which n straight lines divide the plane, Amer. Math. Monthly, 117 (2010), 881888. See Th. 4.
T. Jenkyns and E. Muller, Triangular triples from ceilings to floors, Amer. Math. Monthly, 107 (Aug. 2000), 634639.
V. Jovovic, Vladeta Jovovic, Number of binary matrices
Clark Kimberling and John E. Brown, Partial Complements and Transposable Dispersions, J. Integer Seqs., Vol. 7, 2004.
S. Lafortune, A. Ramani, B. Grammaticos, Y. Ohta and K.M. Tamizhmani, Blending two discrete integrability criteria: ..., arXiv:nlin/0104020 [nlin.SI], 2001.
W. Lanssens, B. Demoen, P.L. Nguyen, The Diagonal Latin Tableau and the Redundancy of its Disequalities, Report CW 666, July 2014, Department of Computer Science, KU Leuven.
S. M. Losanitsch, Die IsomerieArten bei den Homologen der ParaffinReihe, Chem. Ber. 30 (1897), 19171926.
Mircea Merca, Inequalities and Identities Involving Sums of Integer Functions, J. Integer Sequences, Vol. 14 (2011), Article 11.9.1.
Kival Ngaokrajang, Illustration of twin hearts patterns (6c4c): T, U, V
Brian OSullivan and Thomas Busch, Spontaneous emission in ultracold spinpolarised anisotropic Fermi seas, arXiv 0810.0231v1 [quantph], 2008. [Eq 8a, lambda=2]
Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992.
Simon Plouffe, 1031 Generating Functions and Conjectures, Université du Québec à Montréal, 1992.
N. Reading, Order Dimension, Strong Bruhat Order and Lattice Properties for Posets
N. Reading, Order Dimension, Strong Bruhat Order and Lattice Properties for Posets, Order, Vol. 19, no. 1 (2002), 73100.
J. Scholes, 27th Putnam 1966 Prob.A1
N. J. A. Sloane, Classic Sequences
Sam E. Speed, The Integer Sequence A002620 and Upper Antagonistic Functions, Journal of Integer Sequences, Vol. 6 (2003), Article 03.1.4
K. E. Stange, Integral points on elliptic curves and explicit valuations of division polynomials arXiv:1108.3051v3 [math.NT], 20112014.
Thomas Wieder, The number of certain kcombinations of an nset, Applied Mathematics Electronic Notes, vol. 8 (2008).
Wikipedia, Bisymmetric Matrix .
Index entries for twoway infinite sequences
Index entries for linear recurrences with constant coefficients, signature (2,0,2,1).
Index entries for "core" sequences


FORMULA

a(n) = (2*n^21+(1)^(n))/8.  Paul Barry, May 27 2003
G.f.: x^2/((1x)^2*(1x^2)).
E.g.f.: exp(x)*(2*x^2+2*x1)/8+exp(x)/8.
a(n) = 2*a(n1)  2*a(n3) + a(n4).  Jaume Oliver Lafont, Dec 05 2008
a(n) = a(n) for all n in Z.
a(n) = a(n1) + int(n/2), n > 0. Partial sums of A004526.  Adam Kertesz (adamkertesz(AT)worldnet.att.net), Sep 20 2000
a(n) = a(n1) + a(n2)  a(n3) + 1 [with a(1) = a(0) = a(1) = 0], a(2k) = k^2, a(2k1) = k(k1).  Henry Bottomley, Mar 08 2000
0*0, 0*1, 1*1, 1*2, 2*2, 2*3, 3*3, 3*4, ... with an obvious pattern.
a(n) = Sum_{k=1..n} floor(k/2).  Yong Kong (ykong(AT)curagen.com), Mar 10 2001
a(n) = n*floor((n1)/2)  floor((n1)/2)*(floor((n1)/2)+ 1); a(n) = a(n2) + n2 with a(1) = 0, a(2) = 0.  Santi Spadaro, Jul 13 2001
Also: a(n) = binomial(n, 2)  a(n1) = A000217(n1)  a(n1) with a(0) = 0.  Labos Elemer, Apr 26 2003
a(n) = Sum_{k=0..n} (1)^(nk)*C(k, 2).  Paul Barry, Jul 01 2003
a(n) = (1)^n * partial sum of alternating triangular numbers.  Jon Perry, Dec 30 2003
a(n) = A024206(n+1)  n.  Philippe Deléham, Feb 27 2004
a(n) = a(n2) + n  1, n > 1.  Paul Barry, Jul 14 2004
a(n+1) = Sum_{i=0..n} min(i, ni).  Marc LeBrun, Feb 15 2005
a(n+1) = Sum_{k = 0..floor((n1)/2)} n2k; a(n+1) = Sum_{k=0..n} k*(1(1)^(n+k1))/2.  Paul Barry, Apr 16 2005
a(n) = A108561(n+1,n2) for n > 2.  Reinhard Zumkeller, Jun 10 2005
1 + 1/(1 + 2/(1 + 4/(1 + 6/(1 + 9/(1 + 12/(1 + 16/(1 + . . ))))))) = 6/(Pi^2  6) = 1.550546096730...  Philippe Deléham, Jun 20 2005
For n > 2 a(n) = a(n1) + ceiling(sqrt(a(n1))).  Jonathan Vos Post, Jan 19 2006
Sequence starting (2, 2, 4, 6, 9, ...) = A128174 (as an infinite lower triangular matrix) * vector [1, 2, 3, ...]; where A128174 = (1; 0,1; 1,0,1; 0,1,0,1; ...).  Gary W. Adamson, Jul 27 2007
a(n) = Sum_{i=k..n} P(i, k) where P(i, k) is the number of partitions of i into k parts.  Thomas Wieder, Sep 01 2007
a(n) = sum of row (n2) of triangle A115514.  Gary W. Adamson, Oct 25 2007
For n > 1: gcd(a(n+1), a(n)) = a(n+1)  a(n).  Reinhard Zumkeller, Apr 06 2008
a(n+3) = a(n) + A000027(n) + A008619(n+1) = a(n) + A001651(n+1) with a(1) = 0, a(2) = 0, a(3) = 1.  Yosu Yurramendi, Aug 10 2008
a(2n) = A000290(n). a(2n+1) = A002378(n).  Gary W. Adamson, Nov 29 2008
a(n+1) = a(n) + A110654(n).  Reinhard Zumkeller, Aug 06 2009
a(n) = Sum_{k=0..n} (k mod 2)*(nk); Cf. A000035, A001477.  Reinhard Zumkeller, Nov 05 2009
a(n1) = (n*n  2*n + n mod 2)/4.  Ctibor O. Zizka, Nov 23 2009
a(n) = round((2*n^21)/8) = round(n^2/4) = ceiling((n^21)/4).  Mircea Merca, Nov 29 2010
n*a(n+2) = 2*a(n+1) + (n+2)*a(n). Holonomic Ansatz with smallest order of recurrence.  Thotsaporn Thanatipanonda, Dec 12 2010
a(n+1) = (n*(2+n) + n mod 2)/4.  Fred Daniel Kline, Sep 11 2011
a(n) = A199332(n, floor((n+1)/2)).  Reinhard Zumkeller, Nov 23 2011
a(n) = floor(b(n)) with b(n) = b(n1) + n/(1+e^(1/n)) and b(0)= 0.  Richard R. Forberg, Jun 08 2013
a(n) = Sum_{i=1..floor((n+1)/2)} (n+1)2i.  Wesley Ivan Hurt, Jun 09 2013
a(n) = floor((n+2)/2  1)*(floor((n+2)/2)1 + (n+2) mod 2).  Wesley Ivan Hurt, Jun 09 2013
Sum_{n>=2} 1/a(n) = 1 + Zeta(2) = 1+A013661.  Enrique Pérez Herrero, Jun 30 2013
Empirical: a(n) = floor(n/(e^(4/n)1).  Richard R. Forberg, Jul 24 2013
a(n) = A007590(n)/2.  Wesley Ivan Hurt, Mar 08 2014
A240025(a(n)) = 1.  Reinhard Zumkeller, Jul 05 2014
0 = a(n)*a(n+2) + a(n+1)*(2*a(n+2) + a(n+3)) for all integers n.  Michael Somos, Nov 22 2014
a(n) = Sum_{j=1..n} Sum_{i=1..n} ceiling((i+jn1)/2).  Wesley Ivan Hurt, Mar 12 2015
a(4n+1) = A002943(n) for all n>=0.  M. F. Hasler, Oct 11 2015


EXAMPLE

a(3) = 2, floor(3/2)*ceiling(3/2) = 2.
[ n] a(n)

[ 2] 1
[ 3] 2
[ 4] 1 + 3
[ 5] 2 + 4
[ 6] 1 + 3 + 5
[ 7] 2 + 4 + 6
[ 8] 1 + 3 + 5 + 7
[ 9] 2 + 4 + 6 + 8
From Wolfdieter Lang, Dec 09 2014 (Start)
Tiling of a triangular shape T_N, N>=1 with rectangles:
N = 5, n=6: a(6) = 9 because all the rectangles (i, j) (modulo transposition, i.e., interchange of i and j) which are of use are:
(5, 1) ; (1, 1)
(4, 2), (4, 1) ; (2, 2), (2, 1)
; (3, 3), (3, 2), (3, 1)
That is (1+1) + (2+2) + 3 = 9 = a(6). Partial sums of 1, 1, 2, 2, 3, ... (A004526).(End)
Bisymmetric matrices B: 2 X 2, a(3) = 2 from B[1,1] and B[1,2]. 3 X 3, a(4) = 4 from B[1,1], B[1,2], B[1,3], and B[2,2].  Wolfdieter Lang, Jul 07 2015
From John M. Campbell, Jan 29 2016: (Start)
Letting n=5, there are a(n)=a(5)=6 partitions of 2n+1=11 of length three with exactly two even entries:
(8,2,1)  2n+1
(7,2,2)  2n+1
(6,4,1)  2n+1
(6,3,2)  2n+1
(5,4,2)  2n+1
(4,4,3)  2n+1
(End)


MAPLE

A002620 := n>floor(n^2/4); G002620 := series(x^2/((1x)^2*(1x^2)), x, 60);
with(combstruct):ZL:=[st, {st=Prod(left, right), left=Set(U, card=r), right=Set(U, card<r), U=Sequence(Z, card>=1)}, unlabeled]: subs(r=1, stack): seq(count(subs(r=2, ZL), size=m), m=0..57) ; # Zerinvary Lajos, Mar 09 2007
A002620:=1/(z+1)/(z1)^3; # Simon Plouffe in his 1992 dissertation, leading zeros dropped
A002620 := n > add(k, k = select(k > k mod 2 <> n mod 2, [$1 .. n])): seq(A002620(n), n = 0 .. 57);
# Peter Luschny, Jul 06 2011


MATHEMATICA

f[n_] := Ceiling[n/2]Floor[n/2]; Table[ f[n], {n, 0, 56}] (* Robert G. Wilson v, Jun 18 2005 *)
a = 0; Table[(a = n^2 + n  a)/2, {n, 1, 90}] (* Vladimir Joseph Stephan Orlovsky, Nov 18 2009 *)
a[n_] := a[n] = 2a[n  1]  2a[n  3] + a[n  4]; a[0] = a[1] = 0; a[2] = 1; a[3] = 2; Array[a, 60, 0] (* Robert G. Wilson v, Mar 28 2011 *)
LinearRecurrence[{2, 0, 2, 1}, {0, 0, 1, 2}, 60] (* Harvey P. Dale, Oct 05 2012 *)
f[n_] := Block[{c = 0, m = n+1}, Do[ If[ MemberQ[ Range[x, y], (x + y)/2], c++ ], {x, m  1}, {y, x + 1, m}]; c] (* Robert G. Wilson v, May 22 2014 *)


PROG

(MAGMA) [ Floor(n/2)*Ceiling(n/2) : n in [0..40]];
(PARI) a(n)=n^2\4
(PARI) t(n)=n*(n+1)/2 for(i=1, 50, print1(", "(1)^i*sum(k=1, i, (1)^k*t(k))))
(PARI) a(n)=n^2>>2 \\ Charles R Greathouse IV, Nov 11 2009
(Haskell)
a002620 = (`div` 4) . (^ 2)  Reinhard Zumkeller, Feb 24 2012
(Maxima) makelist(floor(n^2/4), n, 0, 50); /* Martin Ettl, Oct 17 2012 */
(PARI) x='x+O('x^100); concat([0, 0], Vec(x^2/((1x)^2*(1x^2)))) \\ Altug Alkan, Oct 15 2015
(Sage)
def A002620():
x, y = 0, 1
yield x
while true:
yield x
x, y = x + y, x//y + 1
a = A002620(); print [a.next() for i in range(58)] # Peter Luschny, Dec 17 2015


CROSSREFS

A087811 is another version of this sequence.
Cf. A024206, A072280, A002984, A007590, A000212, A118015, A056827, A118013, A128174, A000601, A115514, A189151, A063657, A171608, A007590, A005044, A030179, A275437, A004526.
Differences of A002623. Complement of A049068.
a(n) = A014616(n2) + 2 = A033638(n)  1 = A078126(n) + 1. Cf. A055802, A055803.
Antidiagonal sums of array A003983.
Cf. A033436  A033444.  Reinhard Zumkeller, Nov 30 2009
Cf. A008233, A008217, A014980, A197081, A197122.
Elliptic troublemaker sequences: A000212 (= R_n(1,3) = R_n(2,3)), A007590 (= R_n(2,4)), A030511 (= R_n(2,6) = R_n(4,6))), A033436 (= R_n(1,4) = R_n(3,4)), A033437 (= R_n(1,5) = R_n(4,5)), A033438 (= R_n(1,6) = R_n(5,6)), A033439 (= R_n(1,7) = R_n(6,7)), A184535 (= R_n(2,5) = R_n(3,5)).
Cf. A077043, A060656 (2^a(n)).
Sequence in context: A088900 A083392 A076921 * A087811 A025699 A224813
Adjacent sequences: A002617 A002618 A002619 * A002621 A002622 A002623


KEYWORD

nonn,easy,nice,core,changed


AUTHOR

N. J. A. Sloane


STATUS

approved



