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A274921 Spiral constructed on the nodes of the triangular net in which each new term is the least positive integer distinct from its neighbors. 10
1, 2, 3, 2, 3, 2, 3, 1, 3, 1, 2, 1, 3, 1, 2, 1, 3, 1, 2, 3, 2, 1, 2, 3, 1, 3, 2, 1, 2, 3, 1, 3, 2, 1, 2, 3, 1, 2, 1, 3, 2, 3, 1, 2, 3, 2, 1, 3, 2, 3, 1, 2, 3, 2, 1, 3, 2, 3, 1, 2, 3, 1, 3, 2, 1, 3, 1, 2, 3, 1, 2, 1, 3, 2, 1, 3, 1, 2, 3, 1, 2, 1, 3, 2, 1, 3, 1, 2, 3, 1, 2, 3, 2, 1, 3, 2, 1, 2, 3, 1, 2, 3, 1, 3, 2, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

The structure of the spiral has the following properties:

1) Every 1 is surrounded by three equidistant 2's and three equidistant 3's.

2) Every 2 is surrounded by three equidistant 1's and three equidistant 3's.

3) Every 3 is surrounded by three equidistant 1's and three equidistant 2's.

4) Diagonals are periodic sequences with period 3 (A010882 and A130784).

From Juan Pablo Herrera P., Nov 16 2016: (Start)

5) Every hexagon with a 1 in its center is the same hexagon as the one in the middle of the spiral.

6) Every triangle whose number of numbers is divisible by 3 has the same number of 1's, 2's, and 3's. For example, a triangle with 6 numbers, has two 1's, two 2's, and two 3's. (End)

a(n) = a(n-2) if n > 2 is in A014591, otherwise a(n) = 6 - a(n-1)-a(n-2). - Robert Israel, Sep 15 2017

LINKS

Robert Israel, Table of n, a(n) for n = 0..10000

FORMULA

a(n) = A274920(n) + 1.

EXAMPLE

Illustration of initial terms as a spiral:

.

.                3 - 1 - 2 - 3 - 1 - 2

.               /                     \

.              1   2 - 3 - 1 - 2 - 3   1

.             /   /                 \   \

.            2   3   1 - 2 - 3 - 1   2   3

.           /   /   /             \   \   \

.          3   1   2   3 - 1 - 2   3   1   2

.         /   /   /   /         \   \   \   \

.        1   2   3   1   2 - 3   1   2   3   1

.       /   /   /   /   /     \   \   \   \   \

.      2   3   1   2   3   1 - 2   3   1   2   3

.       \   \   \   \   \         /   /   /   /

.        1   2   3   1   2 - 3 - 1   2   3   1

.         \   \   \   \             /   /   /

.          3   1   2   3 - 1 - 2 - 3   1   2

.           \   \   \                 /   /

.            2   3   1 - 2 - 3 - 1 - 2   3

.             \   \                     /

.              1   2 - 3 - 1 - 2 - 3 - 1

.               \

.                3 - 1 - 2 - 3 - 1 - 2

.

MAPLE

A[0]:= 1: A[1]:= 2: A[2]:= 3:

b:= 3: c:= 2: d:= 2: e:= 1: f:= 1:

for n from 3 to 200 do

  if n = b then

     r:= b; b:= c + d - f + 1; f:= e; e:= d; d:= c; c:= r;

     A[n]:= A[n-2];

  else

     A[n]:= 6 - A[n-1] - A[n-2];

  fi

od:

seq(A[i], i=0..200); # Robert Israel, Sep 15 2017

CROSSREFS

Cf. A001399, A010882, A130784, A253186, A274820, A274821, A274920, A275606, A275610.

Sequence in context: A145384 A339601 A117666 * A274821 A275720 A307200

Adjacent sequences:  A274918 A274919 A274920 * A274922 A274923 A274924

KEYWORD

nonn

AUTHOR

Omar E. Pol, Jul 11 2016

STATUS

approved

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Last modified June 22 04:06 EDT 2021. Contains 345367 sequences. (Running on oeis4.)