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Smallest prime factor of n^2 + n - 1.
1

%I #18 Dec 22 2024 12:17:53

%S 5,11,19,29,41,5,71,89,109,131,5,181,11,239,271,5,11,379,419,461,5,19,

%T 599,11,701,5,811,11,929,991,5,19,29,1259,11,5,1481,1559,11,1721,5,31,

%U 1979,2069,2161,5,2351,31,2549

%N Smallest prime factor of n^2 + n - 1.

%H Vincenzo Librandi, <a href="/A209188/b209188.txt">Table of n, a(n) for n = 2..1000</a>

%H PrimeFan, <a href="http://primefan.tripod.com/EsotericIntegerSequences.html">Listing of Esoteric Integer Sequences</a>

%H PrimeFan, <a href="/A031214/a031214.pdf">Esoteric Integer Sequences</a> [Cached copy]

%t SPF[n_] := (FactorInteger[n])[[1]][[1]]; Flatten[Table[SPF[n^2 + (n - 1)], {n, 2, 50}]]

%t Table[FactorInteger[n^2+n-1][[1,1]],{n,2,50}] (* _Harvey P. Dale_, Aug 20 2014 *)

%o (Magma) [Min(PrimeFactors(n^2+n-1)):n in[2..70]];// _Vincenzo Librandi_, Sep 18 2016

%o (PARI) a(n) = vecmin(factor(n^2+n-1)[,1]); \\ _Michel Marcus_, Sep 18 2016

%Y Cf. A071182.

%K nonn

%O 2,1

%A _N. J. A. Sloane_, Mar 05 2012