login
Triangular array of the number of binary, rooted, leaf-labeled tree topologies with n leaves and k cherries, n >= 2, 1 <= k <= floor(n/2).
2

%I #15 Jun 02 2022 16:50:51

%S 1,3,12,3,60,45,360,540,45,2520,6300,1575,20160,75600,37800,1575,

%T 181440,952560,793800,99225,1814400,12700800,15876000,3969000,99225,

%U 19958400,179625600,314344800,130977000,9823275

%N Triangular array of the number of binary, rooted, leaf-labeled tree topologies with n leaves and k cherries, n >= 2, 1 <= k <= floor(n/2).

%C A cherry is an internal node with exactly two descendant leaves. Each binary, rooted, leaf-labeled tree topology with n leaves has at least 1 cherry and at most floor(n/2) cherries.

%H N. A. Rosenberg, <a href="https://doi.org/10.1016/j.aam.2018.09.001">Enumeration of lonely pairs of gene trees and species trees by means of antipodal cherries</a>, Adv. Appl. Math. 102 (2019), 1-17.

%H T. Wu, K. P. Choi, <a href="https://doi.org/10.1016/j.tpb.2015.11.004">On joint subtree distributions under two evolutionary models</a>, Theor. Pop. Biol. 108 (2016), 13-23.

%F T(n,k) = n! (n-2)! / (2^(2k-1) (n-2k)! k! (k-1)! ).

%e For n=4 leaves A, B, C, and D, a(4,1)=12 and a(4,2)=3. The 12 labeled topologies with 1 cherry are (((A,B),C),D), (((A,B),D),C), (((A,C),B),D), (((A,C),D),B), (((A,D),B),C), (((A,D),C),B), (((B,C),A),D), (((B,C),D),A), (((B,D),A),C), (((B,D),C),A), (((C,D),A),B), (((C,D),B),A). The 3 labeled topologies with 2 cherries are ((A,B),(C,D)), ((A,C),(B,D)), ((A,D),(B,C)).

%e Triangular array begins:

%e 1;

%e 3;

%e 12, 3;

%e 60, 45;

%e 360, 540, 45;

%e 2520, 6300, 1575;

%e 20160, 75600, 37800, 1575;

%e 181440, 952560, 793800, 99225;

%e 1814400, 12700800, 15876000, 3969000, 99225;

%e ...

%t Table[n! (n - 2)!/(2^(2 k - 1) (n - 2 k)! k! (k - 1)!), {n, 2, 15}, {k, 1, Floor[n/2]}]

%Y Row sums equal A001147(n-1).

%Y Column k=1 gives A001710.

%Y T(2n,n) gives A079484(n-1).

%K nonn,tabf

%O 2,2

%A _Noah A Rosenberg_, Feb 10 2019