A172294 Sequence of the "Natural Jewels": a natural jewel is a number that is totally enclosed by prime numbers in a version of Ulam Spiral.

12, 42, 1152, 1452, 1950, 3672, 5520, 6660, 8232, 10890, 13218, 15288, 15360, 16062, 18042, 20898, 21018, 23562, 23628, 25998, 27918, 32190, 37812, 42018, 42462, 48858, 55818, 57192, 80832, 80910, 83232, 83340, 91368, 97848, 98640

Offset: 1

Author

Emilio Apricena (emilioapricena(AT)yahoo.it), Jan 30 2010

Keywords

• nonn

Comments

There is no formula to calculate these numbers. If the sequence is infinite, then there are infinitely many primes p such that p + 2 is also prime (twin prime conjecture).

Examples

			11-(top)->12, 13-(left)->12, 29-(right)->12, 31-(bottom)->12.
		

Links

• Emilio Apricena, Illustration

Crossrefs

Cf. A156859.

Extensions

Corrected and extended by Emilio Apricena (emilioapricena(AT)yahoo.it), May 15 2010