A092377 - OEIS

A092377

The O(1) loop model on the square lattice is defined as follows: At every vertex the loop turns to the left or to the right with equal probability, unless the vertex has been visited before, in which case the loop leaves the vertex via the unused edge. Every vertex is visited twice. The probability that a face of the lattice on an n X infinity cylinder is surrounded by five loops is conjectured to be given by a(n)/A_{HT}(n)^2, where A_{HT}(n) is the number of n X n half turn symmetric alternating sign matrices.

1, 1, 956385, 3660540, 3447133563336, 47425519612650, 68120063087909550454, 3225625946195290369800, 8591036125440276726886638297, 1356789922392932853852561183624, 7479333946536834590456926740361593541

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OFFSET

10,3

LINKS

G. C. Greubel, Table of n, a(n) for n = 10..60

Saibal Mitra and Bernard Nienhuis, Osculating Random Walks on Cylinders, in Discrete Random Walks, DRW'03, Cyril Banderier and Christian Krattenthaler (eds.), Discrete Mathematics and Theoretical Computer Science Proceedings AC, pp. 259-264.

FORMULA

Even n: Q(n, m) = C_{n/2-m}(n) + Sum_{r=1..(n-2*m)/4} (-1)^r * ((m+2*r)/(m+r)) * binomial(m+r, r) * C_{n/2-m- 2*r}(n).

Odd n: Q(n, m) = Sum_{r=0..(n-2*m-1)/4)} (-1)^r * binomial(m+r,r) * ( C_{(n-1)/2 -m-2*r}(n) - C_{(n-1)/2 -m-2*r-1}(n) ), where the c_{k}(n) are the absolute values of the coefficients of the characteristic polynomial of the n X n Pascal matrix P_{i, j} = binomial(i+j-2, i-1). The sequence is given by Q(n, 5).

MATHEMATICA

M[n_, k_]:= Table[Binomial[i+j-2, i-1], {i, n}, {j, k}];

c[k_, n_]:= Coefficient[CharacteristicPolynomial[M[n, n], x], x, k]//Abs;

Q[n_?EvenQ, m_]:= Sum[(-1)^r*((m+2*r)/(m+r))*Binomial[m +r, r]*c[n/2 -m-2*r, n], {r, 0, (n-2*m)/4}];

Q[n_?OddQ, m_]:= Sum[(-1)^r*Binomial[m+r, r]*(c[(n-1)/2 -m-2*r, n] - c[(n-1)/2 -m-2*r-1, n]), {r, 0, (n-2*m-1)/4}];

Table[Q[n, 5], {n, 10, 30}] (* G. C. Greubel, Nov 15 2019 *)

CROSSREFS

Cf. A045912, A092372, A092373, A092374, A092375, A092376, A092378, A092379, A092380, A092381, A092382.

Sequence in context: A376473 A341897 A341898 * A348123 A022236 A083609

Adjacent sequences: A092374 A092375 A092376 * A092378 A092379 A092380

KEYWORD

nonn

AUTHOR

Saibal Mitra (smitra(AT)zonnet.nl), Mar 20 2004

STATUS

approved