OFFSET
1,9
COMMENTS
Take the standard counterclockwise square spiral starting at 0, as in A304586, but only write one digit at a time in the cells of the spiral: 0,1,2,3,4,5,6,7,8,9,1,0,1,1,1,2,...
Place a chess knight at cell 0. Move it to the lowest-numbered cell it can attack, and if there is a tie, move it to the cell closest (in Euclidean distance) to the start, and if there is still a tie, move to the left(*).
No cell can be visited more than once.
Inspired by the Trapped Knight video and A316667.
Just as for A316667, the sequence is finite. After a while, the knight has no unvisited squares it can reach, and the sequence ends with a(1217) = 4.
(*)Moving to the left means choose the point with the lowest x-coordinate. This leads to an unambiguous choice of tied squares only for the 'move left' case.
LINKS
Luca Petrone, Table of n, a(n) for n = 1..1217
Eric Angelini, Kneil's Knumberphile Knight, Cinquante signes, May 04 2019.
M. F. Hasler, Knight tours, OEIS wiki, Nov. 2019.
N. J. A. Sloane and Brady Haran, The Trapped Knight, Numberphile video (2019).
EXAMPLE
The digit-square spiral is
.
.
2---2---2---1---2---0---2 2
| | |
3 1---2---1---1---1 9 3
| | | | |
2 3 4---3---2 0 1 1
| | | | | | |
4 1 5 0---1 1 8 3
| | | | | |
2 4 6---7---8---9 1 0
| | | |
5 1---5---1---6---1---7 3
| |
2---6---2---7---2---8---2---9
CROSSREFS
KEYWORD
nonn,fini,full
AUTHOR
N. J. A. Sloane, Oct 17 2019
EXTENSIONS
More terms from Luca Petrone
Corrected and extended by Eric Angelini, Oct 24 2019
STATUS
approved