

A109613


Odd numbers repeated.


76



1, 1, 3, 3, 5, 5, 7, 7, 9, 9, 11, 11, 13, 13, 15, 15, 17, 17, 19, 19, 21, 21, 23, 23, 25, 25, 27, 27, 29, 29, 31, 31, 33, 33, 35, 35, 37, 37, 39, 39, 41, 41, 43, 43, 45, 45, 47, 47, 49, 49, 51, 51, 53, 53, 55, 55, 57, 57, 59, 59, 61, 61, 63, 63, 65, 65, 67, 67, 69, 69, 71, 71, 73
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

0,3


COMMENTS

The number of rounds in a roundrobin tournament with n competitors.  A. Timothy Royappa, Aug 13 2011
a(n) = A052928(n) + 1 = 2*A004526(n) + 1.
a(n) = A028242(n) + A110654(n).
Diagonal sums of number triangle A113126.  Paul Barry, Oct 14 2005
When partitioning a convex ngon by all the diagonals, the maximum number of sides in resulting polygons is 2*floor(n/2)+1 = a(n1) (from Moscow Olympiad problem 1950).  Tanya Khovanova, Apr 06 2008
Its ordinal transform is A000034.  Paolo P. Lava, Jun 25 2009
The inverse values of the coefficients in the series expansion of f(x) = (1/2)*(1+x)*log((1+x)/(1x)) lead to this sequence; cf. A098557.  Johannes W. Meijer, Nov 12 2009
From Reinhard Zumkeller, Dec 05 2009: (Start)
First differences: A010673; partial sums: A000982;
A059329(n) = Sum_{k = 0..n} a(k)*a(nk);
A167875(n) = Sum_{k = 0..n} a(k)*A005408(nk);
A171218(n) = Sum_{k = 0..n} a(k)*A005843(nk);
A008794(n+2) = Sum_{k = 0..n} a(k)*A059841(nk). (End)
Dimension of the space of weight 2n+4 cusp forms for Gamma_0(5).  Michael Somos, May 29 2013
For n > 4: a(n) = A230584(n)  A230584(n2).  Reinhard Zumkeller, Feb 10 2015
The arithmetic function v+(n,2) as defined in A290988.  Robert Price, Aug 22 2017
For n > 0, also the chromatic number of the (n+1)triangular (Johnson) graph.  Eric W. Weisstein, Nov 17 2017
a(n1), for n >= 1, is also the upper bound a_{up}(b), where b = 2*n + 1, in the first (top) row of the complete coach system Sigma(b) of Hilton and Pedersen [HP]. All odd numbers <= a_{up}(b) of the smallest positive restricted residue system of b appear once in the first rows of the c(2*n+1) = A135303(n) coaches. If b is an odd prime a_{up}(b) is the maximum. See a comment in the proof of the quasiorder theorem of HP, on page 263 ["Furthermore, every possible a_i < b/2 ..."]. For an example see below.  Wolfdieter Lang, Feb 19 2020


REFERENCES

Peter Hilton and Jean Pedersen, A Mathematical Tapestry: Demonstrating the Beautiful Unity of Mathematics, Cambridge University Press, 2010, 3rd printing 2012, pp. (260281).


LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 0..10000
Eric Weisstein's World of Mathematics, Chromatic Number
Eric Weisstein's World of Mathematics, Johnson Graph
Eric Weisstein's World of Mathematics, Triangular Graph
Wikipedia, Roundrobin tournament
Index entries for linear recurrences with constant coefficients, signature (1,1,1).


FORMULA

a(n) = 2*floor(n/2) + 1.
a(n) = A052938(n2) + A084964(n2) for n > 1.  Reinhard Zumkeller, Aug 27 2005
G.f.: (1 + x + x^2 + x^3)/(1  x^2)^2.  Paul Barry, Oct 14 2005
a(n) = n + (1 + (1)^n)/2.  Paolo P. Lava, May 08 2007
a(n) = 2*a(n2)  a(n4), a(0) = 1, a(1) = 1, a(2) = 3, a(3) = 3.  Philippe Deléham, Nov 03 2008
a(n) = A001477(n) + A059841(n).  Philippe Deléham, Mar 31 2009
a(n) = 2*n  a(n1), with a(0) = 1.  Vincenzo Librandi, Nov 13 2010
a(n) = R(n, 2), where R(n, x) is the nth row polynomial of A211955. a(n) = (1)^n + 2*Sum_{k = 1..n} (1)^(n  k  2)*4^(k1)*binomial(n+k, 2*k). Cf. A084159.  Peter Bala, May 01 2012
a(n) = A182579(n+1, n).  Reinhard Zumkeller, May 06 2012
G.f.: ( 1 + x^2 ) / ( (1 + x)*(x  1)^2 ).  R. J. Mathar, Jul 12 2016
E.g.f.: x*exp(x) + cosh(x).  Ilya Gutkovskiy, Jul 12 2016
From Guenther Schrack, Sep 10 2018: (Start)
a(n) = a(n1).
a(n) = A047270(n+1)  (2*n + 2).
a(n) = A005408(A004526(n)). (End)


EXAMPLE

G.f. = 1 + x + 3*x^2 + 3*x^3 + 5*x^4 + 5*x^5 + 7*x^6 + 7*x^7 + 9*x^8 + 9*x^9 + ...
Complete coach system for (a composite) b = 2*n + 1 = 33: Sigma(33) ={[1; 5], [5, 7, 13; 2, 1, 2]} (the first two rows are here 1 and 5, 7, 13), a_{up}(33) = a(15) = 15. But 15 is not in the reduced residue system modulo 33, so the maximal (odd) a number is 13. For the prime b = 31, a_{up}(31) = a(14) = 15 appears as maximum of the first rows.  Wolfdieter Lang, Feb 19 2020


MAPLE

A109613:=n>2*floor(n/2)+1; seq(A109613(k), k=0..100); # Wesley Ivan Hurt, Oct 22 2013


MATHEMATICA

Flatten@ Array[{2#  1, 2#  1} &, 37] (* Robert G. Wilson v, Jul 07 2012 *)
(#  Boole[EvenQ[#]] &) /@ Range[80] (* Alonso del Arte, Sep 11 2019 *)
With[{c=2*Range[0, 40]+1}, Riffle[c, c]] (* Harvey P. Dale, Jan 02 2020 *)


PROG

(Haskell)
a109613 = (+ 1) . (* 2) . (`div` 2)
a109613_list = 1 : 1 : map (+ 2) a109613_list
 Reinhard Zumkeller, Oct 27 2012, Feb 21 2011
(PARI) A109613(n)=n>>1<<1+1 \\ Charles R Greathouse IV, Feb 24, 2011
(Sage) def a(n) : return( len( CuspForms( Gamma0( 5), 2*n + 4, prec=1). basis())); # Michael Somos, May 29 2013
(Scala) ((1 to 49) by 2) flatMap { List.fill(2)(_) } // Alonso del Arte, Sep 11 2019


CROSSREFS

Cf. A063196, A110660, A186421, A186422, A211955, A230584, A290988.
Complement of A052928 with respect to the universe A004526.  Guenther Schrack, Aug 21 2018
First differences of A000982, A061925, A074148, A105343, A116940, and A179207.  Guenther Schrack, Aug 21 2018
Sequence in context: A296063 A127630 A267458 * A063196 A339110 A245150
Adjacent sequences: A109610 A109611 A109612 * A109614 A109615 A109616


KEYWORD

nonn,easy


AUTHOR

Reinhard Zumkeller, Aug 01 2005


STATUS

approved



