OFFSET
1,2
COMMENTS
All terms are multiples of 12.
LINKS
Shalosh B. Ekhad, More terms.
Rebecca Embar and Doron Zeilberger, Counting Condorcet.
Doron Zeilberger, Condorcet3 Maple package.
FORMULA
a(n) = (4*(19*n^2-57*n+45)/(n-1)^2)*a(n-1) - (36*(2*n-3)*(22*n^2-99*n+111)/((n-2)*(n-1)^2))*a(n-2) + (1296*(n-3)*(2*n-3)*(2*n-5)/((n-2)*(n-1)^2))*a(n-3).
a(n) = 2*(Sum_{i1=0..n-2} Sum_{i2=0..n-2-i1} Sum_{i3=0..n-2-i1-i2} Sum_{i4=0..n-2-i1-i2-i3} Sum_{i5=0..n-2-i1-i2-i3-i4} ((2*n-1)!/((n-1-i2-i3-i5)!*i2!*i3!*(i2+i4+i5+1)!*(n-1-i1-i2-i4)!*i1!))).
a(n) ~ (1/4 - 3*arcsin(1/3)/(2*Pi)) * 6^(2*n - 1) [Guilbaud, 1952].
MAPLE
#(From Maple package Condorcet3 by Doron Zeilberger)
#NuCo(N):The first N terms of the sequence "number of Condorcet vote-profiles" with 2v-1 voters and three candidates. Using the third-order recurrence. Try:
#NuCo(100);
NuCo:=proc(N) local L, n, kha: L:=[0, 12, 540]: if N<=3 then RETURN(L[N]): fi: for n from 4 to N do kha:=4*(19*n^2-57*n+45)/(n-1)^2*L[-1]-36*(2*n-3)*(22*n^2-99*n+111)/(n-2)/(n-1)^2*L[-2]+1296*(n-3)*(2*n-3)*(2*n-5)/(n-2)/(n-1)^2*L[-3]: L:=[L[2], L[3], kha]: od: L[-1]: end: seq(NuCo(n), n=1..16);
MATHEMATICA
RecurrenceTable[{a[n] == (4*(19*n^2 - 57*n + 45)/(n-1)^2) * a[n-1] - (36*(2*n - 3)*(22*n^2 - 99*n + 111)/((n-2)*(n-1)^2)) * a[n-2] + (1296*(n-3)*(2*n - 3)*(2*n - 5)/((n-2)*(n-1)^2)) * a[n-3], a[1] == 0, a[2] == 12, a[3] == 540}, a[n], {n, 1, 20}] (* Vaclav Kotesovec, May 20 2022 *)
PROG
(PARI) a(n) = 2*sum(i1=0, n-2, sum(i2=0, n-2-i1, sum(i3=0, n-2-i1-i2, sum(i4=0, n-2-i1-i2-i3, sum(i5=0, n-2-i1-i2-i3-i4, ((2*n-1)!/((n-1-i2-i3-i5)!*i2!*i3!*(i2+i4+i5+1)!*(n-1-i1-i2-i4)!*i1!))))))) \\ Michel Marcus, May 03 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Rebecca Embar, Apr 29 2022
STATUS
approved