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A299916 a(n) = A299914(2n+1). 3
1, 6, 42, 306, 2250, 16578, 122202, 900882, 6641514, 48963042, 360969210, 2661166386, 19618866954, 144635805954, 1066295850138, 7861032979794, 57953746616490, 427251323790882, 3149816954720058, 23221336706989938, 171194226906268746, 1262092001672539458 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

a(n) is the number of holes shaped like six-pointed stars, in descending size, found in the cross-section, in the shape of a regular hexagon, of a Menger Sponge. - Albert Säfström, Jul 25 2018

REFERENCES

Murat Sahin and Elif Tan, Conditional (strong) divisibility sequences, Fib. Q., 56 (No. 1, 2018), 18-31.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..200

Albert Säfström, Illustration of regular hexagonal cross-section of Menger Sponge, with supporting triangular shape

Index entries for linear recurrences with constant coefficients, signature (9,-12)

FORMULA

G.f.: -(3*x-1)/(12*x^2-9*x+1). - Alois P. Heinz, Mar 10 2018

From Colin Barker, Mar 12 2018: (Start)

a(n) = 2^(-1-n)*((9-sqrt(33))^n*(-3+sqrt(33)) + (3+sqrt(33))*(9+sqrt(33))^n) / sqrt(33).

a(n) = 9*a(n-1) - 12*a(n-2) for n>1.

(End)

MAPLE

a:= n-> (<<0|1>, <-12|9>>^n. <<1, 6>>)[1, 1]:

seq(a(n), n=0..25);  # Alois P. Heinz, Mar 10 2018

MATHEMATICA

CoefficientList[Series[-(3 x - 1)/(12 x^2 - 9 x + 1), {x, 0, 20}], x] (* Michael De Vlieger, Mar 10 2018 *)

LinearRecurrence[{9, -12}, {1, 6}, 30] (* Vincenzo Librandi, Mar 11 2018 *)

PROG

(MAGMA) I:=[1, 6]; [n le 2 select I[n] else 9*Self(n-1)-12*Self(n-2): n in [1..30]]; // Vincenzo Librandi, Mar 11 2018

(PARI) Vec((1 - 3*x) / (1 - 9*x + 12*x^2) + O(x^30)) \\ Colin Barker, Mar 12 2018

CROSSREFS

Cf. A299914.

Sequence in context: A147838 A127628 A111602 * A091164 A004982 A093388

Adjacent sequences:  A299913 A299914 A299915 * A299917 A299918 A299919

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane, Mar 10 2018

EXTENSIONS

More terms from Altug Alkan, Mar 10 2018

STATUS

approved

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Last modified November 20 21:02 EST 2018. Contains 317422 sequences. (Running on oeis4.)