A339849 - OEIS

A339849

Square array T(n,k), n >= 2, k >= 2, read by antidiagonals, where T(n,k) is the number of Hamiltonian circuits within parallelograms of size n X k on the triangular lattice.

%I #34 Dec 26 2020 02:42:01

%S 1,1,1,1,4,1,1,13,13,1,1,44,80,44,1,1,148,549,549,148,1,1,498,3851,

%T 7104,3851,498,1,1,1676,26499,104100,104100,26499,1676,1,1,5640,

%U 183521,1475286,3292184,1475286,183521,5640,1,1,18980,1269684,20842802,100766213,100766213,20842802,1269684,18980,1

%N Square array T(n,k), n >= 2, k >= 2, read by antidiagonals, where T(n,k) is the number of Hamiltonian circuits within parallelograms of size n X k on the triangular lattice.

%H Seiichi Manyama, <a href="/A339849/b339849.txt">Antidiagonals n = 2..13, flattened</a>

%H M. Peto, <a href="https://doi.org/10.31274/rtd-180813-17105">Studies of protein designability using reduced models</a>, Thesis, 2007.

%F T(n,k) = T(k,n).

%e Square array T(n,k) begins:

%e 1, 1, 1, 1, 1, 1, ...

%e 1, 4, 13, 44, 148, 498, ...

%e 1, 13, 80, 549, 3851, 26499, ...

%e 1, 44, 549, 7104, 104100, 1475286, ...

%e 1, 148, 3851, 104100, 3292184, 100766213, ...

%e 1, 498, 26499, 1475286, 100766213, 6523266332, ...

%o (Python)

%o # Using graphillion

%o from graphillion import GraphSet

%o def make_T_nk(n, k):

%o grids = []

%o for i in range(1, k + 1):

%o for j in range(1, n):

%o grids.append((i + (j - 1) * k, i + j * k))

%o if i < k:

%o grids.append((i + (j - 1) * k, i + j * k + 1))

%o for i in range(1, k * n, k):

%o for j in range(1, k):

%o grids.append((i + j - 1, i + j))

%o return grids

%o def A339849(n, k):

%o universe = make_T_nk(n, k)

%o GraphSet.set_universe(universe)

%o cycles = GraphSet.cycles(is_hamilton=True)

%o return cycles.len()

%o print([A339849(j + 2, i - j + 2) for i in range(11 - 1) for j in range(i + 1)])

%Y Rows and columns 3..10 give A339850, A339851, A339852, A338970, A339622, A339960, A339961, A339962.

%Y Main diagonal gives A339854.

%Y Cf. A339190.

%K nonn,tabl

%O 2,5

%A _Seiichi Manyama_, Dec 19 2020