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A214457 Table read by antidiagonals in which entry T(n,k) in row n and column k gives the number of possible rhombus tilings of an octagon with interior angles of 135 degrees and sequences of side lengths {n, k, 1, 1, n, k, 1, 1} (as the octagon is traversed), n,k in {1,2,3,...}. 1
8, 20, 20, 40, 75, 40, 70, 210, 210, 70, 112, 490, 784, 490, 112, 168, 1008, 2352, 2352, 1008, 168, 240, 1890, 6048, 8820, 6048, 1890, 240, 330, 3300, 13860, 27720, 27720, 13860, 3300, 330, 440, 5445, 29040, 76230, 104544, 76230, 29040, 5445, 440 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Proof of the formula for T(n,k) is given in [Elnitsky].

So-called "generalized Narayana numbers" (see  A145596), linking rhombus tilings of polygons to certain walks or paths through the square lattice.

(Start) Table begins as

...8....20....40.....70....112...;

..20....75...210....490...1008...;

..40...210...784...2352...6048...;

..70...490..2352...8820..27720...;

.112..1008..6048..27720..76230...;

etc. (End)

LINKS

Table of n, a(n) for n=1..45.

Serge Elnitsky, Rhombic tilings of polygons and classes of reduced words in Coxeter groups (preprint), J. Combin. Theory Ser. A, Vol. 77, Issue 2, 193-221 (1997).

L. E. Jeffery, Worked out example for A214457(1,1)=8

FORMULA

T(n,k) = 2*(n+k+1)!*(n+k+2)!/[n!*k!*(n+2)!*(k+2)!].

EXAMPLE

See [Jeffery]. T(1,1) = 8 because there are eight ways to tile the proposed octagon with rhombuses.

CROSSREFS

Empirical: T(1,n) = T(n,1) = 2*A000292(n+1); T(2,n) = T(n,2) = A006411(n+1); T(n,n) = A145600(n+1).

Sequence in context: A115147 A302241 A022700 * A205226 A205318 A100212

Adjacent sequences:  A214454 A214455 A214456 * A214458 A214459 A214460

KEYWORD

nonn,tabl

AUTHOR

L. Edson Jeffery, Jul 18 2012

STATUS

approved

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Last modified November 14 07:19 EST 2019. Contains 329111 sequences. (Running on oeis4.)