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A045610 Number of different energy states of n positive and n negative charges on a string. 2
1, 1, 3, 7, 22, 70, 249, 880, 3238, 12180, 46247, 174458, 672920, 2585414, 10015955 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Table of n, a(n) for n=0..14.

Sean A. Irvine, Java program, GitHub.

EXAMPLE

For n=3 the 20 different arrangements of -1,-1,-1,1,1,1 result in 7 energy levels (sum of signed inverse distances):

{0,0,0,1,1,1},{1,1,1,0,0,0}: 13/10

{0,0,1,0,1,1},{1,1,0,1,0,0}: -41/30

{0,0,1,1,0,1},{0,1,0,0,1,1},{1,0,1,1,0,0},{1,1,0,0,1,0}: -56/30

{0,0,1,1,1,0},{0,1,1,1,0,0},{1,0,0,0,1,1},{1,1,0,0,0,1}: -8/10

{0,1,0,1,0,1},{1,0,1,0,1,0}: -37/10

{0,1,0,1,1,0},{0,1,1,0,1,0},{1,0,0,1,0,1},{1,0,1,0,0,1}: -89/30

{0,1,1,0,0,1},{1,0,0,1,1,0}: -71/30

so the multiplicities are 4*2 + 3*4 = 20 = binomial(6,3).

MATHEMATICA

f[li_: {(0 | 1) ..}] := Outer[Times, 2 li - 1, 2 li - 1];

Table[Length @ Tally[Total[1/DeleteCases[f[#] DistanceMatrix[Range[2 n]], 0, 2], 2] & /@ Permutations[Join[Table[0, n], Table[1, n]]]], {n, 10}] (* Wouter Meeussen, Mar 15 2021 *)

CROSSREFS

Cf. A045723.

Sequence in context: A319312 A325213 A148688 * A259809 A340022 A181769

Adjacent sequences:  A045607 A045608 A045609 * A045611 A045612 A045613

KEYWORD

nonn,more

AUTHOR

Wouter Meeussen

EXTENSIONS

Corrected and extended by Wouter Meeussen, Mar 15 2021

a(12)-a(15) from Sean A. Irvine, Mar 15 2021

STATUS

approved

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Last modified May 19 05:04 EDT 2022. Contains 353826 sequences. (Running on oeis4.)