

A004526


Nonnegative integers repeated, floor(n/2).


271



0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 23, 24, 24, 25, 25, 26, 26, 27, 27, 28, 28, 29, 29, 30, 30, 31, 31, 32, 32, 33, 33, 34, 34, 35, 35, 36, 36
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OFFSET

0,5


COMMENTS

Number of elements in the set {k: 1 <= 2k <= n}.
Dimension of the space of weight 2n+4 cusp forms for Gamma_0(2).
Dimension of the space of weight 1 modular forms for Gamma_1(n+1).
Number of ways 2^n is expressible as r^2s^2 with s > 0. Proof: (r+s) and (rs) both should be powers of 2, even and distinct hence a(2k) = a(2k1) = (k1) etc.  Amarnath Murthy, Sep 20 2002
Lengths of sides of Ulam square spiral; i.e., lengths of runs of equal terms in A063826.  Donald S. McDonald, Jan 09 2003
Number of partitions of n into two parts. A008619 gives partitions of n into at most two parts, so A008619(n) = A004526(n) + 1 for all n >= 0. Partial sums are A002620 (Quartersquares).  Rick L. Shepherd, Feb 27 2004
a(n+1) is the number of 1's in the binary expansion of the Jacobsthal number A001045(n).  Paul Barry, Jan 13 2005
Partitions of n+1 into two distinct (nonzero) parts. Example: a(8) = 4 because we have [8,1],[7,2],[6,3] and [5,4].  Emeric Deutsch, Apr 14 2006
Complement of A000035, since A000035(n)+2*a(n) = n. Also equal to the partial sums of A000035.  Hieronymus Fischer, Jun 01 2007
Number of binary bracelets of n beads, two of them 0. For n >= 2, a(n2) is the number of binary bracelets of n beads, two of them 0, with 00 prohibited.  Washington Bomfim, Aug 27 2008
Let A be the Hessenberg n by n matrix defined by: A[1,j] = j mod 2, A[i,i]:=1, A[i,i1] = 1, and A[i,j] = 0 otherwise. Then, for n >= 1, a(n+1) = (1)^n det(A).  Milan Janjic, Jan 24 2010
Let RT abbreviate rank transform (A187224). Then
RT(A004526)=A187484;
RT(A004526 without 1st term)=A026371;
RT(A004526 without 1st 2 terms)=A026367;
RT(A004526 without 1st 3 terms)=A026363.  Clark Kimberling, Mar 10 2011
The diameter (longest path) of the ncycle.  Cade Herron, Apr 14 2011
For n >= 3, a(n1) is the number of twocolor bracelets of n beads, three of them are black, having a diameter of symmetry.  Vladimir Shevelev, May 03 2011
Pelesko (2004) refers erroneously to this sequence instead of A008619.  M. F. Hasler, Jul 19 2012
Number of degree 2 irreducible characters of the dihedral group of order 2(n+1).  Eric M. Schmidt, Feb 12 2013
For n >= 3 the sequence a(n1) is the number of noncongruent regions with infinite area in the exterior of a regular ngon with all diagonals drawn. See A217748.  Martin Renner, Mar 23 2013
a(n) is the number of partitions of 2n into exactly 2 even parts. a(n+1) is the number of partitions of 2n into exactly 2 odd parts. This just rephrases the comment of E. Deutsch above.  Wesley Ivan Hurt, Jun 08 2013
Number of the distinct rectangles and square in a regular ngon is A004526(n/2) for even n and n >= 4. For odd n, such number is zero, see illustration in link.  Kival Ngaokrajang, Jun 25 2013
xcoordinate from the image of the point (0,1) after n reflections across the lines y = n and y = x respectively (alternating so that one reflection is applied on each step): (0,1) > (0,1) > (1,0) > (1,2) > (2,1) > (2,3) > ... .  Wesley Ivan Hurt, Jul 12 2013
a(n) is the number of partitions of 2n into exactly two distinct odd parts. a(n1) is the number of partitions of 2n into exactly two distinct even parts, n > 0.  Wesley Ivan Hurt, Jul 21 2013
a(n) is the number of permutations of length n avoiding 213, 231 and 312, or avoiding 213, 312 and 321 in the classical sense which are breadthfirst search reading words of increasing unarybinary trees. For more details, see the entry for permutations avoiding 231 at A245898.  Manda Riehl, Aug 05 2014
Also a(n) is the number of different patterns of 2color, 2partition of n.  Ctibor O. Zizka, Nov 19 2014
Minimum in and out degree for a directed K_n (see link).  Jon Perry, Nov 22 2014
a(n) is also the independence number of the triangular graph T(n).  Luis Manuel Rivera MartÃnez, Mar 12 2015


REFERENCES

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.
L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 120, P(n,2).
Graham, Knuth and Patashnik, Concrete Mathematics, AddisonWesley, NY, 1989, page 77 (partitions of n into at most 2 parts).


LINKS

David Wasserman, Table of n, a(n) for n = 0..1000
Kival Ngaokrajang, The distinct rectangles and square in a regular ngon for n = 4..18
John A. Pelesko, Generalizing the ConwayHofstadter $10,000 Sequence, Journal of Integer Sequences, Vol. 7 (2004), Article 04.3.5.
Jon Perry, Square of a directed graph.
William A. Stein, Dimensions of the spaces S_k(Gamma_0(N))
William A. Stein, The modular forms database
Eric Weisstein's World of Mathematics, Prime Partition
Index entries for "core" sequences
Index entries for linear recurrences with constant coefficients, signature (1,1,1).


FORMULA

G.f.: x^2/((1+x)*(x1)^2).
a(n) = floor(n/2).
a(n) = 1 + a(n2).
a(n) = a(n1) + a(n2)  a(n3).
a(2n) = a(2n+1) = n.
a(n+1) = na(n).  Henry Bottomley, Jul 25 2001
For n > 0, a(n) = sum(i=1, n, (1/2)/cos(Pi*(2*i(1(1)^n)/2)/(2*n+1))).  Benoit Cloitre, Oct 11 2002
a(n) = (2n1)/4+(1)^n/4; a(n+1) = sum{k=0..n, k*(1)^(n+k)}.  Paul Barry, May 20 2003
E.g.f.: ((2x1)exp(x)+exp(x))/4.  Paul Barry, Sep 03 2003
G.f.: 1/(1x) * sum(k >= 0, t^2/(1t^4), t = x^2^k).  Ralf Stephan, Feb 24 2004
a(n+1) = A000120(A001045(n)).  Paul Barry, Jan 13 2005
a(n) = (n(1(1)^n)/2)/2 = 1/2*(nsin(n*Pi/2)). Likewise: a(n) = (nA000035(n))/2. Also: a(n) = sum{0<=k<=n, A000035(k)}.  Hieronymus Fischer, Jun 01 2007
The expression floor((x^21)/(2*x)) (x >= 1) produces this sequence.  Mohammad K. Azarian, Nov 08 2007; corrected by M. F. Hasler, Nov 17 2008
a(n+1) = A002378(n)  A035608(n).  Reinhard Zumkeller, Jan 27 2010
a(n+1) = A002620(n+1)  A002620(n) = floor((n+1)/2)*ceiling((n+1)/2)  floor(n^2/4).  Jonathan Vos Post, May 20 2010
For n >= 2, a(n) = floor(log_2(2^a(n1)+2^a(n2))).  Vladimir Shevelev, Jun 22 2010
a(n) = A180969(2,n).  Adriano Caroli, Nov 24 2010
A001057(n1) = (1)^n*a(n), n > 0.  M. F. Hasler, Jul 19 2012
a(n) = A008615(n) + A002264(n).  Reinhard Zumkeller, Apr 28 2014
Euler transform of length 2 sequence [ 1, 1].  Michael Somos, Jul 03 2014


EXAMPLE

G.f. = x^2 + x^3 + 2*x^4 + 2*x^5 + 3*x^6 + 3*x^7 + 4*x^8 + 4*x^9 + 5*x^10 + ...


MAPLE

A004526 := n>floor(n/2); seq(floor(i/2), i=0..50);


MATHEMATICA

Table[(2n  1)/4 + (1)^n/4, {n, 0, 70}] (* Stefan Steinerberger, Apr 02 2006 *)
f[n_] := If[OddQ[n], (n  1)/2, n/2]; Array[f, 74, 0] (* Robert G. Wilson v, Apr 20 2012 *)
With[{c=Range[0, 40]}, Riffle[c, c]] (* Harvey P. Dale, Aug 26 2013 *)
CoefficientList[Series[x^2/(1  x  x^2 + x^3), {x, 0, 75}], x] (* Robert G. Wilson v, Feb 05 2015 *)
LinearRecurrence[{1, 1, 1}, {0, 0, 1}, 75] (* Robert G. Wilson v, Feb 05 2015 *)


PROG

(PARI) a(n)=n\2 /* Jaume Oliver Lafont, Mar 25 2009 */
(Haskell)
a004526 = (`div` 2)
a004526_list = concatMap (\x > [x, x]) [0..]
 Reinhard Zumkeller, Jul 27 2012
(Maxima) makelist(floor(n/2), n, 0, 50); /* Martin Ettl, Oct 17 2012 */
(Sage) def a(n) : return( dimension_cusp_forms( Gamma0(2), 2*n+4) ); # Michael Somos, Jul 03 2014
(Sage) def a(n) : return( dimension_modular_forms( Gamma1(n+1), 1) ); # Michael Somos, Jul 03 2014
(MAGMA) [Floor(n/2): n in [0..100]]; // Vincenzo Librandi, Nov 19 2014


CROSSREFS

a(n+2) = A008619(n). See A008619 for more references.
A001477(n) = A004526(n+1)+A004526(n). A000035(n) = A004526(n+1)A002456(n).
a(n) = A008284(n, 2), n >= 1.
Zero followed by the partial sums of A000035.
Column 2 of triangle A094953. Second row of A180969.
Cf. A002264, A002265, A002266, A010761, A010762, A110532, A110533.
Partial sums: A002620. Other related sequences: A010872, A010873, A010874.
Sequence in context: A001057 A130472 A076938 * A140106 A123108 A008619
Adjacent sequences: A004523 A004524 A004525 * A004527 A004528 A004529


KEYWORD

nonn,easy,core,nice


AUTHOR

N. J. A. Sloane


EXTENSIONS

Partially edited by Joerg Arndt, Mar 11 2010, and M. F. Hasler, Jul 19 2012


STATUS

approved



