A369314 - OEIS

A369314

Number of chiral pairs of polyominoes composed of n triangular cells of the hyperbolic regular tiling with Schläfli symbol {3,oo}.

1, 2, 7, 22, 68, 214, 691, 2240, 7396, 24702, 83469, 284928, 981814, 3410990, 11939752, 42075308, 149180356, 531866972, 1905872189, 6861162880, 24805796984, 90035940942, 327988261992, 1198853954688, 4395798528850

(list; graph; refs; listen; history; text; internal format)

OFFSET

4,2

COMMENTS

A stereographic projection of the {3,oo} tiling on the Poincaré disk can be obtained via the Christensson link. Each member of a chiral pair is a reflection but not a rotation of the other.

LINKS

Table of n, a(n) for n=4..28.

Malin Christensson, Make hyperbolic tilings of images, web page, 2019.

FORMULA

a(n) = C(2n,2)/(2(n+1)(n+2)) - [2\(n+1)]*C(n,(n+1)/2)/(2n) - [2\n]*C(n,n/2)/(2n+4) + [3\(n-1)]*C((2n+1)/3,(n-1)/3)/(2n+1).

a(n) = A001683(n+2) - A000207(n) = (A001683(n+2) - A208355(n-1)) / 2 = A000207(n) - A208355(n-1).

EXAMPLE

________ ________ ________ ________ ________ ________

\ /\ /\ /\ /\ / \ /\ /\ /\ /\ / \ /\ /\ /\ /\ /

\/__\/__\ /__\/__\/ \/__\/__\ /__\/__\/ \/__\/__\ /__\/__\/

\ / \ / \ / \ /

a(4)=1; a(5)=2. \/ \/ \/ \/

MATHEMATICA

Table[Binomial[2n, n]/(2(n+1)(n+2))-If[OddQ[n], Binomial[n, (n+1)/2]/n, Binomial[n, n/2]/(n+2)]/2+If[Divisible[n-1, 3], Binomial[(2n+1)/3, (n-1)/3]/(2n+1), 0], {n, 4, 20}]

CROSSREFS

Polyominoes: A001683(n+2) (oriented), A000207 (unoriented), A208355(n-1) (achiral).

Sequence in context: A037552 A308113 A291012 * A294005 A333494 A292399

Adjacent sequences: A369311 A369312 A369313 * A369315 A369316 A369317

KEYWORD

nonn,easy

AUTHOR

Robert A. Russell, Jan 19 2024

STATUS

approved