OFFSET
1,1
COMMENTS
The equilateral triangle composed of 144 smaller equilateral triangles is the smallest triangle that can be tiled with the sphinx. This triangle is used to form all orders of the hexagon.
Walter Trump enumerated all 830 sphinx tilings of this triangle and found six symmetrical examples one of which is used to produce this sequence.
Hyper-packing is a term that describes the ability of a shape to contain a greater area of subshapes than its own area by overlapping the subshapes. There are 864 unit triangles in the order 1 hexagon. 30 of the subshapes hyper-packed into this hexagon would contain 30x6x6 or 1080 unit triangles if summed individually.
The prime numbers cannot be described by a formula. Subsets of the primes such as the balanced primes are more formula friendly (see comments to puzzle 920 below). - Craig Knecht, Apr 19 2018
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..5000
Craig Knecht, Example for the sequence.
Craig Knecht, Order 4 Hexagon with 246 subshapes.
Craig Knecht, Sphinx Tile Component Triangles.
Craig Knecht, Sphinx tiling of the triangle used to make the hexagon.
Carlos Rivera, Puzzle 920. An enigma related to A291582, Primes puzzles and problems connections.
Wikipedia, Hyper-Packing the Sphinx.
Wikipedia, Symmetric sphinx tiled triangles.
Wikipedia, Walter Trump.
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = 6*n*(6*n-1). - Walter Trump
G.f.: 2*x*(15+21*x)/(1-x)^3. - Vincenzo Librandi, Sep 20 2017
E.g.f.: 6*exp(x)*(5 + 17*x + 6*x^2). - Stefano Spezia, Dec 07 2018
From Amiram Eldar, Jul 30 2024: (Start)
Sum_{n>=1} 1/a(n) = log(2)/3 + log(3)/4 - sqrt(3)*Pi/12.
Sum_{n>=1} (-1)^(n+1)/a(n) = Pi/6 - log(2)/6 - arccoth(sqrt(3))/sqrt(3). (End)
MAPLE
seq(6*n*(6*n-1), n=1..100); # Robert Israel, Sep 19 2017
MATHEMATICA
Array[6 # (6 # - 1) &, 42] (* Michael De Vlieger, Sep 19 2017 *)
CoefficientList[Series[2(15 + 21 x)/(1-x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Sep 20 2017 *)
CoefficientList[Series[6 E^x (5 + 17 x + 6 x^2), {x, 0, 50}], x]*
Table[n!, {n, 0, 50}] (* Stefano Spezia, Dec 07 2018 *)
PROG
(Magma) [6*n*(6*n-1): n in [1..50]]; // Vincenzo Librandi, Sep 20 2017
(PARI) a(n) = 6*n*(6*n-1); \\ Altug Alkan, Apr 08 2018
(Sage) [6*n*(6*n-1) for n in (1..50)] # G. C. Greubel, Dec 04 2018
(GAP) List([1..30], n -> 6*n*(6*n-1)); # G. C. Greubel, Dec 04 2018
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Craig Knecht, Aug 30 2017
STATUS
approved