A257321 Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "2" arm.

2, 5, 7, 5, 3, 11, 5, 2, 7, 2, 3, 2, 5, 2, 5, 7, 3, 5, 7, 2, 5, 7, 11, 3, 7, 3, 5, 3, 5, 3, 5, 3, 7, 3, 2, 5, 2, 7, 2, 3, 5, 7, 2, 7, 2, 7, 2, 7, 2, 5, 11, 2, 5, 2, 3, 2, 5, 2, 5, 7, 3, 7, 3, 7, 3, 5, 3, 5, 3, 5, 7, 2, 5, 7, 3, 7, 3, 7, 3, 2, 5, 2, 7, 2, 3, 5

Offset: 1

Author

N. J. A. Sloane, Apr 21 2015

Keywords

• nonn

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.

Repetitions in arms are permitted.

The four arms are A257321, A257322, A257323, A257324.

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

Cf. A064413, A257322-A257340.

Extensions

More terms from Lars Blomberg, Apr 27 2015