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%I #24 Nov 05 2017 02:15:28
%S 5,17,37,5,101,5,197,257,5,401,5,577,677,5,17,5,13,1297,5,1601,5,13,
%T 29,5,41,5,2917,3137,5,13,5,17,4357,5,13,5,5477,53,5,37,5,7057,13,5,
%U 8101,5,8837,13,5,73,5,29,17,5,12101,5,41,13457,5
%N Smallest prime divisor of 4*n^2+1.
%C a(n) = A020639(A053755(n)).
%C If the map "x -> smallest odd prime divisor of n^2+1" is iterated, does it always terminate in the 2-cycle (5 <-> 13)? - _Zoran Sunic_, Oct 25 2017
%D Richard Friedberg, An Adventurer's Guide to Number Theory, McGraw-Hill, NY, 1968.
%D Popular Computing (Calabasas, CA), Friedberg's Sequence, Vol. 5 (No. 46, Jan 1977), page PC46-2.
%H Reinhard Zumkeller, <a href="/A256970/b256970.txt">Table of n, a(n) for n = 1..10000</a>
%t Table[FactorInteger[4*n^2+1][[1,1]],{n,59}] (* _Ivan N. Ianakiev_, Apr 20 2015 *)
%o (Haskell)
%o a256970 = a020639 . a053755 -- _Reinhard Zumkeller_, Apr 20 2015
%o (PARI) a(n) = factor(4*n^2+1)[1,1]; \\ _Michel Marcus_, Apr 20 2015
%Y Cf. A053755, A256971, A020639.
%Y A bisection of A125256.
%K nonn
%O 1,1
%A _N. J. A. Sloane_, Apr 19 2015
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