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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. 1
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

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).

LINKS

Table of n, a(n) for n=1..35.

Emilio Apricena, Illustration

EXAMPLE

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

CROSSREFS

Cf. A156859.

Sequence in context: A334277 A122973 A074356 * A238225 A088826 A125221

Adjacent sequences:  A172291 A172292 A172293 * A172295 A172296 A172297

KEYWORD

nonn

AUTHOR

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

EXTENSIONS

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

STATUS

approved

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Last modified June 6 03:35 EDT 2020. Contains 334858 sequences. (Running on oeis4.)