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%I #7 Oct 05 2019 16:34:39
%S 12,42,1152,1452,1950,3672,5520,6660,8232,10890,13218,15288,15360,
%T 16062,18042,20898,21018,23562,23628,25998,27918,32190,37812,42018,
%U 42462,48858,55818,57192,80832,80910,83232,83340,91368,97848,98640
%N Sequence of the "Natural Jewels": a natural jewel is a number that is totally enclosed by prime numbers in a version of Ulam Spiral.
%C 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).
%H Emilio Apricena, <a href="/A172294/a172294.png">Illustration</a>
%e 11-(top)->12, 13-(left)->12, 29-(right)->12, 31-(bottom)->12.
%Y Cf. A156859.
%K nonn
%O 1,1
%A Emilio Apricena (emilioapricena(AT)yahoo.it), Jan 30 2010
%E Corrected and extended by Emilio Apricena (emilioapricena(AT)yahoo.it), May 15 2010
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