

A008621


Expansion of 1/((1x)*(1x^4)).


20



1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11, 11, 11, 12, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21
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OFFSET

0,5


COMMENTS

Arises from Gleason's theorem on selfdual codes: 1/((1x^2)*(1x^8)) is the Molien series for the real 2dimensional Clifford group (a dihedral group of order 16) of genus 1.
Count of odd numbers between consecutive quartersquares, A002620. Oppermann's conjecture states that for each count there will be at least one prime.  Fred Daniel Kline, Sep 10 2011
Number of partitions into parts 1 and 4.  Joerg Arndt, Jun 01 2013


REFERENCES

D. J. Benson, Polynomial Invariants of Finite Groups, Cambridge, 1993, p. 100.
F. J. MacWilliams and N. J. A. Sloane, Theory of ErrorCorrecting Codes, 1977, Chapter 19, Problem 3, p. 602.


LINKS

T. D. Noe, Table of n, a(n) for n=0..1000
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 211
G. Nebe, E. M. Rains and N. J. A. Sloane, SelfDual Codes and Invariant Theory, Springer, Berlin, 2006.
Index entries for Molien series
Index entries for linear recurrences with constant coefficients, signature (1,0,0,1,1).
Wikipedia, Oppermann's Conjecture


FORMULA

a(n) = floor((n+3)/4), n>0;
a(n) = A010766(n+4, 4).
a(n) = {Sum{k=0..n, (k+1)cos(pi*(nk)/2}+1/4[cos(n*Pi/2)+1+(1)^n] }/2  Paolo P. Lava, Oct 09 2006
Also, a(n) = ceiling((n+1)/4), n >= 0.  Mohammad K. Azarian, May 22 2007
a(n) = Sum_{i=0..n} A121262(i) = n/4 +5/8 +(1)^n/8 + A057077(n)/4.  R. J. Mathar, Mar 14 2011
a(x,y):= floor(x/2) + floor(y/2)  x where x=A002620(n) and y=A002620(n+1), n>2.  Fred Daniel Kline, Sep 10 2011
a(0)=1, a(1)=1, a(2)=1, a(3)=1, a(4)=2, a(n) = a(n1) + a(n4)  a(n5).  Harvey P. Dale, Feb 19 2012


MATHEMATICA

Table[Floor[(n + 3)/4], {n, 1, 80}] (* Stefan Steinerberger, Apr 03 2006 *)
CoefficientList[Series[1/((1x)(1x^4)), {x, 0, 80}], x] (* Harvey P. Dale, Feb 19 2012 *)
Flatten[ Table[ PadRight[{}, 4, n], {n, 19}]] (* Harvey P. Dale, Feb 19 2012 *)


CROSSREFS

Cf. A008718, A024186, A110160, A110868, A110869, A110876, A110880.
Cf. A002265, A008620.
Sequence in context: A242601 A110655 * A144075 A128929 A257839 A075245
Adjacent sequences: A008618 A008619 A008620 * A008622 A008623 A008624


KEYWORD

nonn,easy,nice


AUTHOR

N. J. A. Sloane


EXTENSIONS

More terms from Stefan Steinerberger, Apr 03 2006


STATUS

approved



