A257328 Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "7" arm.

7, 3, 11, 9, 2, 13, 4, 5, 8, 17, 19, 10, 21, 23, 25, 29, 31, 35, 37, 15, 41, 43, 47, 16, 53, 20, 59, 22, 49, 61, 27, 14, 55, 26, 67, 28, 33, 65, 32, 71, 34, 73, 38, 79, 44, 83, 39, 77, 51, 89, 91, 57, 85, 97, 40, 63, 101, 103, 107, 109, 113, 95, 127, 115, 6

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. Inside an arm, the terms must be distinct.

Repetitions between arms are permitted.

The four arms are A257325, A257326, A257327, A257328.

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, A257321-A257340.

Sequence in context: A168197 A257324 A138859 * A361416 A076569 A061194

Adjacent sequences: A257325 A257326 A257327 * A257329 A257330 A257331

Keyword

nonn

Author

N. J. A. Sloane, Apr 21 2015

Extensions

More terms from Lars Blomberg, Apr 27 2015

Status

approved