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%I #12 Aug 08 2020 02:21:17
%S 5,3,73,3,5,3,7,3,5,3,13,3,5,3,19,3,5,3,7,3,5,3,13,3,5,3,37,3,5,3,7,3,
%T 5,3,13,3,5,3,71,3,5,3,7,3,5,3,13,3,5,3,19,3,5,3,7,3,5,3,13,3,5,3,37,
%U 3,5,3,7,3,5,3,13,3,5,3,73,3,5,3,7,3,5,3
%N Smallest prime factor of 1+78557*2^n, cf. A258073.
%C a(n) = A020639(A258073(a(n)));
%C a(n) <= 73; see also A258095.
%C Periodic, a(n) = a(n + 840420) for all n (and 840420 is minimal with this property). The only values that occur, are {3, 5, 7, 13, 19, 37, 73} union {47, 59, 71}. - _Jeppe Stig Nielsen_, Jul 19 2020
%H Reinhard Zumkeller, <a href="/A258091/b258091.txt">Table of n, a(n) for n = 1..10000</a>
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Sierpinski_number">Sierpinski Number</a>
%o (Haskell) a258091 = a020639 . a258073
%o (PARI) a(n)=forprime(p=2,,78557*Mod(2,p)^n+1==0 && return(p)) \\ _Jeppe Stig Nielsen_, Jul 19 2020
%Y Cf. A020639, A258073, A258095.
%K nonn,easy
%O 1,1
%A _Reinhard Zumkeller_, May 19 2015
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