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A274820 Spiral constructed on the nodes of the infinite triangular net in which each term is the least nonnegative integer such that no diagonal contains a repeated term. 18
0, 1, 2, 1, 2, 1, 2, 0, 3, 0, 4, 3, 5, 3, 4, 5, 3, 4, 6, 5, 6, 7, 4, 6, 5, 7, 6, 3, 0, 6, 5, 7, 0, 6, 7, 5, 4, 8, 1, 3, 6, 8, 1, 9, 7, 8, 2, 4, 9, 8, 2, 10, 11, 8, 9, 10, 12, 3, 8, 9, 7, 10, 9, 2, 4, 8, 5, 10, 2, 11, 9, 11, 0, 10, 7, 8, 6, 0, 9, 7, 10, 12, 7, 1, 4, 8, 5, 11, 1, 10, 12, 9, 5, 11, 10, 13, 12, 11, 13, 14 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Also spiral constructed on the infinite hexagonal grid in which each term is the least nonnegative integer such that no diagonal of successive adjacent cells contains a repeated term. Every number is located in the center of a hexagonal cell. Every cell is also the center of three diagonals of successive adjacent cells.

Presumably every line of cells with slope a multiple of 60 degrees (not necessarily passing through the central cell) is a permutation of the nonnegative numbers. See A296343-A296348 for the spokes through the central cell. - N. J. A. Sloane, Dec 12 2017

LINKS

Rémy Sigrist, Table of n, a(n) for n = 0..120400

F. Michel Dekking, Jeffrey Shallit, and N. J. A. Sloane, Queens in exile: non-attacking queens on infinite chess boards, arXiv:1907.09120, July 2019

Rémy Sigrist, PARI program for A274820

Rémy Sigrist, Colored illustration of the first 200 windings of the spiral (where the color is a function of a(n))

N. J. A. Sloane, Illustration of initial terms drawn as a spiral on the hexagonal grid (the starting cell is marked in black).

FORMULA

a(n) = A274821(n) - 1.

EXAMPLE

Illustration of initial terms as a spiral:

.

.                   9 - 4 - 2 - 8 - 7

.                  /                 \

.                 8   3 - 6 - 7 - 5   9

.                /   /             \   \

.               2   0   5 - 3 - 4   6   1

.              /   /   /         \   \   \

.            10   6   3   1 - 2   0   4   8

.            /   /   /   /     \   \   \   \

.          11   5   4   2   0 - 1   3   7   6

.            \   \   \   \         /   /   /

.             8   7   5   1 - 2 - 0   6   3

.              \   \   \             /   /

.               9   0   3 - 4 - 6 - 5   1

.                \   \                 /

.                10   6 - 7 - 5 - 4 - 8

.                  \

.                  12 - 3 - 8 - 9 - 7

.

PROG

(PARI) See Links section.

CROSSREFS

Cf. A001477, A269526, A274528 (square array), A274641 (spiral on the square grid), A274650 (right triangle), A274821, A274920, A274921, A275606, A275610, A296339.

A296342 says when n first appears.

See A296343-A296348 for the spokes.

Sequence in context: A029445 A274920 A316828 * A230583 A197366 A245715

Adjacent sequences:  A274817 A274818 A274819 * A274821 A274822 A274823

KEYWORD

nonn,look

AUTHOR

Omar E. Pol, Jul 09 2016

STATUS

approved

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Last modified January 19 03:18 EST 2020. Contains 331031 sequences. (Running on oeis4.)