cp's OEIS Frontend

A185121 Smallest prime factor of 10^(2^n) + 1.

%I A185121 #58 Aug 15 2023 13:26:53
%S A185121 11,101,73,17,353,19841,1265011073,257,10753,1514497,
%T A185121 1856104284667693057,106907803649,458924033,
%U A185121 3635898263938497962802538435084289
%N A185121 Smallest prime factor of 10^(2^n) + 1.
%C A185121 10^k+1 can only be prime if k is a power of 2. So far the only known primes of this form are a(0) = 11 and a(1) = 101. [Edited by _M. F. Hasler_, Aug 03 2019]
%C A185121 a(n) >= 2^(n+1)+1; we have a(n) = 2^(n+1)+1 for n=3, n=7, and n=15.
%C A185121 a(14) > 10^16. - _Max Alekseyev_, Jun 28 2013
%C A185121 From the Keller link a(15)-a(20) = 65537, 8257537, 175636481, 639631361, 70254593, 167772161. - _Ray Chandler_, Dec 27 2013
%H A185121 FactorDB, <a href="http://factordb.com/index.php?query=10^(2^n)%2B1&amp;use=n&amp;n=1">Factorizations of 10^(2^n)+1</a>
%H A185121 Makoto Kamada, <a href="https://stdkmd.net/nrr/repunit/10001.htm">Factorizations of 100...001</a>
%H A185121 Wilfrid Keller, <a href="http://web.archive.org/web/20160617002823/http://www.prothsearch.net/GFN10.html">Prime factors of generalized Fermat numbers Fm(10) and complete factoring status</a>
%F A185121 a(n) = A038371(2^n). - _M. F. Hasler_, Jul 30 2019
%e A185121 For n=2, a(2)=73 since 10^(2^2) + 1 = 10001 = 73 * 137.
%t A185121 Table[With[{k = 2^n}, FactorInteger[10^k + 1]][[1, 1]], {n, 0, 13, 1}] (* _Vincenzo Librandi_, Jul 23 2013 *)
%o A185121 (PARI) a(n) = factor(10^(2^n)+1)[1, 1] \\ _Michel Marcus_, May 30 2013
%Y A185121 Cf. A038371, A000533, A000215, A093179, A102050
%Y A185121 Essentially the same as A102050. - _Sean A. Irvine_, Feb 17 2013
%K A185121 nonn,more,hard
%O A185121 0,1
%A A185121 _Sergio Pimentel_, Jan 22 2012