OFFSET
1,1
COMMENTS
Place numbers 2,3,5,7 clockwise around a grid point (see illustrations in links). Divide grid into four spiral arms.
Extend each arm one step at a time, in rotation: first the 2 arm, then the 3 arm, then the 5 arm, then the 7 arm, then the 2 arm, etc.
Rule for extending: next term in arm is smallest number such that each cell in the grid is relatively prime to its eight neighbors. Every term in the entire grid must be different.
Conjecture: every number > 1 appears in one of the four arms.
LINKS
Lars Blomberg, Table of n, a(n) for n = 1..10000
Popular Computing (Calabasas, CA), Problem 146: Gcd, Vol. 4 (No. 45, Dec 1976), page PC45-4.
N. J. A. Sloane, Spirals showing initial terms of A257321-A257332
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 21 2015
EXTENSIONS
More terms from Lars Blomberg, Apr 27 2015
STATUS
approved