OFFSET
0,2
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Eryk Kopczyński, Dorota Celińska and Marek Čtrnáct, HyperRogue: Playing with Hyperbolic Geometry, Proceedings of Bridges 2017: Mathematics, Art, Music, Architecture, Education, Culture, Pages 9-16
This sequence is important in the game HyperRogue which uses this tiling.
Wikipedia, Truncated order-7 triangular tiling
Index entries for linear recurrences with constant coefficients, signature (1,1,1,-1).
FORMULA
a(n+4) = a(n+3) + a(n+2) + a(n+1) - a(n), for n >= 1. (proved)
G.f.: (1 + 6*x + 6*x^2 + 6*x^3 + x^4) / (1 - x - x^2 - x^3 + x^4). - Colin Barker, Jan 05 2018
EXAMPLE
There is only the original heptagon in distance 0, so a(0)=1. It is adjacent to 7 hexagons, so a(1)=7. These are adjacent to 7 new heptagons and 7 new hexagons, so a(2)=14.
MATHEMATICA
LinearRecurrence[{1, 1, 1, -1}, {1, 7, 14, 28, 49}, 40] (* Harvey P. Dale, Mar 25 2019 *)
PROG
(PARI) Vec((1 + 6*x + 6*x^2 + 6*x^3 + x^4) / (1 - x - x^2 - x^3 + x^4) + O(x^40)) \\ Colin Barker, Jan 05 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Eryk Kopczynski, Jul 29 2017
STATUS
approved