

A008621


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


19



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 to sequences with 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 [From 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. A008620, A002265.
Sequence in context: A002265 A110655 * A144075 A128929 A075245 A129253
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



