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A145393 Number of inequivalent sublattices of index n in square lattice, where two sublattices are considered equivalent if one can be rotated or reflected to give the other, with that rotation or reflection preserving the parent square lattice. 19
1, 2, 2, 4, 3, 5, 3, 7, 5, 7, 4, 11, 5, 8, 8, 12, 6, 13, 6, 15, 10, 11, 7, 21, 10, 13, 12, 18, 9, 22, 9, 21, 14, 16, 14, 29, 11, 17, 16, 29, 12, 28, 12, 25, 23, 20, 13, 39, 16, 27, 20, 29, 15, 34, 20, 36, 22, 25, 16, 50, 17, 26, 29, 38, 24, 40, 18, 36, 26, 40 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

From Andrey Zabolotskiy, Mar 12 2018: (Start)

If reflections are not allowed, we get A145392. If any rotations and reflections are allowed, we get A054346.

The parent lattice of the sublattices under consideration has Patterson symmetry group p4mm, and two sublattices are considered equivalent if they are related via a symmetry from that group [Rutherford]. For other 2D Patterson groups, the analogous sequences are A000203 (p2), A069734 (p2mm), A145391 (c2mm), A145392 (p4), A145394 (p6), A003051 (p6mm).

Rutherford says at p. 161 that a(n) != A054346(n) only when A002654(n) > 2, but actually these two sequence differ at other terms, too, for example, at n = 30 (see illustration). (End)

LINKS

Andrey Zabolotskiy, Table of n, a(n) for n = 1..10000

Amihay Hanany, Domenico Orlando, and Susanne Reffert, Sublattice counting and orbifolds, High Energ. Phys., 2010 (2010), 51, arXiv.org:1002.2981 [hep-th] (see table 6 and fig. 2).

John S. Rutherford, Sublattice enumeration. IV. Equivalence classes of plane sublattices by parent Patterson symmetry and colour lattice group type, Acta Cryst. (2009). A65, 156-163. [See Table 2; beware the typo in a(5).]

Andrey Zabolotskiy, Sublattices of the square lattice (illustrations for n = 1..6, 15, 25)

Index entries for sequences related to sublattices

Index entries for sequences related to square lattice

FORMULA

a(n) = (A000203(n) + A002654(n) + A069735(n) + A145390(n))/4. [Rutherford] - N. J. A. Sloane, Mar 13 2009

G.f.: Sum_{ m>=1 } (1/((1-x^m)(1-x^(4m))) - 1). [Hanany, Orlando & Reffert, eq. (6.8)] - Andrey Zabolotskiy, Jul 05 2017

a(n) = Sum_{ m: m^2|n } A019590(n/m^2) + A157228(n/m^2) + A157226(n/m^2) + A157230(n/m^2) + A157231(n/m^2) = A053866(n) + A025441(n) + Sum_{ m: m^2|n } A157226(n/m^2) + A157230(n/m^2) + A157231(n/m^2). [Rutherford] - Andrey Zabolotskiy, May 07 2018

a(n) = Sum_{ d|n } A008621(d) = Sum_{ d|n } (1 + floor(d/4)). [From the above-given g.f.] - Andrey Zabolotskiy, Jul 17 2019

MATHEMATICA

terms = 70;

CoefficientList[Sum[(1/((1-x^m)(1-x^(4m)))-1), {m, 1, terms}] + O[x]^(terms + 1), x] // Rest (* Jean-Fran├žois Alcover, Aug 05 2018 *)

CROSSREFS

Cf. A054345, A054346, A167156, A008621.

Cf. A000203, A069734, A145391, A145392, A145394, A003051, A002324, A002654, A069735, A145390.

Cf. A019590, A157228, A157226, A157230, A157231, A053866, A025441.

Sequence in context: A086671 A269502 A054346 * A215675 A329439 A132802

Adjacent sequences:  A145390 A145391 A145392 * A145394 A145395 A145396

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Feb 23 2009

EXTENSIONS

New name from Andrey Zabolotskiy, Mar 12 2018

STATUS

approved

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Last modified December 12 07:00 EST 2019. Contains 329948 sequences. (Running on oeis4.)