

A274879


A statistic on orbital systems over n sectors: the number of orbitals with k returns.


10



1, 1, 2, 2, 4, 2, 4, 6, 12, 12, 4, 8, 8, 20, 40, 48, 32, 10, 20, 24, 16, 70, 140, 180, 160, 80, 28, 56, 72, 64, 32, 252, 504, 672, 672, 480, 192, 84, 168, 224, 224, 160, 64, 924, 1848, 2520, 2688, 2240, 1344, 448, 264, 528, 720, 768, 640, 384, 128
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OFFSET

0,3


COMMENTS

The definition of an orbital system is given in A232500 (see also the illustration there). The number of orbitals over n sectors is counted by the swinging factorial A056040.
When a segment of an orbital starts at a point on the central circle this point is called a 'return' of the orbital if it is not the origin.
If an orbital touches the central circle only in the origin it is called a prime orbital. Column 0 counts the prime orbitals over n sectors.
A108747 is a subtriangle.


LINKS

Table of n, a(n) for n=0..56.
Peter Luschny, Orbitals


FORMULA

For even n>0: T(n,k) = 2^(k+1)*(k+1)*binomial(nk1,n/2)/(nk1) for k=0..n/21 (from A108747).


EXAMPLE

Triangle read by rows, n>=0. The length of row n is floor((n+1)/2) for n>=1.
[ n] [k=0,1,2,...] [row sum]
[ 0] [1] 1
[ 1] [1] 1
[ 2] [2] 2
[ 3] [2, 4] 6
[ 4] [2, 4] 6
[ 5] [6, 12, 12] 30
[ 6] [4, 8, 8] 20
[ 7] [20, 40, 48, 32] 140
[ 8] [10, 20, 24, 16] 70
[ 9] [70, 140, 180, 160, 80] 630
[10] [28, 56, 72, 64, 32] 252
[11] [252, 504, 672, 672, 480, 192] 2772
T(6,0) = 4 because the following 4 orbitals stay above or below the central
circle: [1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1],
[1, 1, 1, 1, 1, 1].


PROG

(Sage)
# Brute force counting, function unit_orbitals defined in A274709.
def orbital_returns(n):
if n == 0: return [1]
S = [0]*((n+1)//2)
for u in unit_orbitals(n):
L = list(accumulate(u))
Z = len(filter(lambda z: z == 0, L))
S[Z1] += 1 # exclude origin
return S
for n in (0..10): print orbital_returns(n)


CROSSREFS

Cf. A056040 (row sum), A108747, A232500, A241543 (col. 0).
Other orbital statistics: A241477 (first zero crossing), A274706 (absolute integral), A274708 (peaks), A274709 (max. height), A274710 (number of turns), A274878 (span), A274880 (restarts), A274881 (ascent).
Sequence in context: A035096 A066675 A219433 * A222043 A222153 A219403
Adjacent sequences: A274876 A274877 A274878 * A274880 A274881 A274882


KEYWORD

nonn,tabf


AUTHOR

Peter Luschny, Jul 11 2016


STATUS

approved



