A257339 Arrange numbers in a single clockwise spiral so that each number is relatively prime to its eight neighbors.

1, 2, 3, 5, 4, 7, 6, 11, 9, 13, 17, 19, 8, 23, 29, 21, 25, 31, 37, 41, 35, 43, 47, 10, 49, 12, 53, 14, 15, 59, 27, 55, 39, 16, 61, 22, 51, 67, 18, 65, 24, 71, 26, 33, 32, 57, 73, 79, 83, 77, 85, 89, 97, 101, 28, 95, 34, 91, 38, 103, 107, 45, 109, 113, 20, 121

Offset: 1

Author

N. J. A. Sloane, Apr 21 2015

Keywords

• nonn

Comments

Start with 1; always choose smallest number which has not yet appeared.

It is conjectured that every number appears.

Links

• Jon E. Schoenfield and Reinhard Zumkeller, Table of n, a(n) for n = 1..100000 (First 10000 terms from Jon E. Schoenfield)
• Jon E. Schoenfield, Illustration of initial terms
• Jon E. Schoenfield, Graph of first 18000 terms

Crossrefs

Cf. A064413, A257321-A257340.

Cf. A257111 (first differences), A257455 (putative inverse).

Indices of primes and prime powers: A257457 and A257458.

Extensions

Corrected and extended by Jon E. Schoenfield, Apr 23 2015