A208828 Number of 7-bead necklaces labeled with numbers -n..n allowing reversal, with sum zero.

32, 609, 4390, 19657, 65134, 177097, 417204, 883409, 1720628, 3135633, 5410378, 8921049, 14153874, 21728505, 32416136, 47167393, 67132584, 93694273, 128489486, 173447145, 230812054, 303187369, 393560188, 505349233, 642431868, 809197169

Offset: 1

Author

R. H. Hardin, Mar 01 2012

Keywords

• nonn

Comments

Row 7 of A208825.

Examples

			Some solutions for n=3:
  -3  -3  -2  -3  -3  -3  -3  -3  -3  -3  -3  -2  -3  -1  -3  -2
   1  -1  -2  -1   2  -2   1  -1  -2  -2   3  -2   2  -1  -1  -1
  -1   3   0  -2   0   2  -1   1   1  -1   0  -1  -2   0  -1   1
  -1   1   1  -1   1   1   2   1   3   2  -2  -1   0   0   1  -1
   1   0   2   2  -2   2  -3   1  -3   2  -2   3   1   1   1  -1
   2  -2  -1   3  -1  -2   1   2   3   3   1   0   0   1   2   3
   1   2   2   2   3   2   3  -1   1  -1   3   3   2   0   1   1
		

Links

• R. H. Hardin, Table of n, a(n) for n = 1..210

Crossrefs

Cf. A208825.

Formula

Empirical: a(n) = 4*a(n-1) - 3*a(n-2) - 8*a(n-3) + 14*a(n-4) - 14*a(n-6) + 8*a(n-7) + 3*a(n-8) - 4*a(n-9) + a(n-10).

Empirical g.f.: x*(32 + 481*x + 2050*x^2 + 4180*x^3 + 4100*x^4 + 2126*x^5 + 462*x^6 + 28*x^7 - 4*x^8 + x^9) / ((1 - x)^7*(1 + x)^3). - Colin Barker, Jul 07 2018