A268172 - OEIS

%I #35 Sep 11 2017 16:38:51

%S 0,1,1,2,4,9,23,58,156,426,1194,3393,9802,28601,84347,250732,750908,

%T 2262817,6857386,20882889,63877262,196162762,604567254,1869318719,

%U 5797113028,18026873112,56197262814,175594836698,549839459963,1725126992844,5422602630117,17074281639963,53848886560675,170085320026578

%N Binary-ternary Wedderburn-Etherington numbers.

%C This is the number of non-planar binary-ternary rooted trees (every node has out-degree 0 or 2 or 3) with n leaf nodes, indexed by the number of leaf nodes (NOT the total number of nodes).

%C It can also be interpreted as the number of bracketings (valid placements of operation symbols) in a monomial of degree n in a nonassociative algebra with an (anti-)commutative binary operation and a completely (skew-)symmetric ternary operation.

%H Alois P. Heinz, <a href="/A268172/b268172.txt">Table of n, a(n) for n = 0..1000</a>

%H Murray R. Bremner, <a href="/A268172/a268172.txt">Maple code for binary-ternary Wedderburn-Etherington numbers</a>

%H Murray R. Bremner, <a href="/A268172/a268172.pdf">Recursion formula for binary-ternary Wedderburn-Etherington numbers</a>

%F See Maple code, and the recursion formula under Links.

%e Here are the 1, 1, 2, 4, 9, 23 bracketings for degrees 1 to 6 (using the monomial interpretation), where the binary and ternary operations are written [-,-] and [-,-,-] respectively, and the hyphen is a placeholder for the argument symbols:

%e Degree 1: -.

%e Degree 2: [-,-].

%e Degree 3: [[-,-],-], [-,-,-].

%e Degree 4: [[[-,-],-],-], [[-,-],[-,-]], [[-,-,-],-], [[-,-],-,-].

%e Degree 5:

%e    [[[[-,-],-],-],-],

%e    [[[-,-,-],-],-],

%e    [[[-,-],[-,-]],-],

%e    [[[-,-],-,-],-],

%e    [[[-,-],-],[-,-]],

%e    [[-,-,-],[-,-]],

%e    [[[-,-],-],-,-],

%e    [[-,-,-],-,-],

%e    [[-,-],[-,-],-].

%e Degree 6:

%e    [[[[[-,-],-],-],-],-],

%e    [[[[-,-,-],-],-],-],

%e    [[[[-,-],[-,-]],-],-],

%e    [[[[-,-],-,-],-],-],

%e    [[[[-,-],-],[-,-]],-],

%e    [[[-,-,-],[-,-]],-],

%e    [[[[-,-],-],-,-],-],

%e    [[[-,-,-],-,-],-],

%e    [[[-,-], [-,-],-],-],

%e    [[[[-,-],-],-],[-,-]],

%e    [[[-,-,-],-],[-,-]],

%e    [[[-,-], [-,-]],[-,-]],

%e    [[[-,-],-,-],[-,-]],

%e    [[[-,-],-],[[-,-],-]],

%e    [[[-,-],-],[-,-,-]],

%e    [[-,-,-],[-,-,-]],

%e    [[[[-,-],-],-],-,-],

%e    [[[-,-,-],-],-,-],

%e    [[[-,-],[-,-]],-,-],

%e    [[[-,-],-,-],-,-],

%e    [[[-,-],-],[-,-],-],

%e    [[-,-,-],[-,-],-],

%e    [[-,-],[-,-],[-,-]].

%p # for first Maple program see Links

%p # second Maple program:

%p b:= proc(n, i, v) option remember; `if`(n=0,

%p       `if`(v=0, 1, 0), `if`(i<1 or v<1 or n<v, 0,

%p       `if`(v=n, 1, add(binomial(a(i)+j-1, j)*

%p        b(n-i*j, i-1, v-j), j=0..min(n/i, v)))))

%p     end:

%p a:= proc(n) option remember; `if`(n<2, n,

%p       add(b(n, n+1-j, j), j=2..3))

%p     end:

%p seq(a(n), n=0..40);  # _Alois P. Heinz_, Jan 28 2016

%t b[n_, i_, v_] := b[n, i, v] = If[n==0, If[v==0, 1, 0], If[i<1 || v<1 || n<v, 0, If[v==n, 1, Sum[Binomial[a[i]+j-1, j]*b[n-i*j, i-1, v-j], {j, 0, Min[n/i, v]}]]]]; a[n_] := a[n] = If[n<2, n, Sum[b[n, n+1-j, j], {j, 2, 3}]]; Table[a[n], {n, 0, 40}] (* _Jean-François Alcover_, Feb 25 2017, after _Alois P. Heinz_ *)

%Y Cf. A001190 (Binary Wedderburn-Etherington numbers).

%Y Cf. A000598 (Ternary Wedderburn-Etherington numbers: number of non-planar ternary rooted trees with n nodes): note that this sequence is indexed by the total number of nodes, NOT the number of leaves.

%Y Cf. A000669, A268163.

%Y Column k=3 of A292085.

%K easy,nonn

%O 0,4

%A _Murray R. Bremner_, Jan 27 2016