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 A069901 Smallest prime factor of n-th triangular number. 7

%I

%S 1,3,2,2,3,3,2,2,3,5,2,2,7,3,2,2,3,3,2,2,3,11,2,2,5,3,2,2,3,3,2,2,3,5,

%T 2,2,19,3,2,2,3,3,2,2,3,23,2,2,5,3,2,2,3,3,2,2,3,29,2,2,31,3,2,2,3,3,

%U 2,2,3,5,2,2,37,3,2,2,3,3,2,2,3,41,2,2,5,3,2,2,3,3,2,2,3,5,2,2,7,3,2

%N Smallest prime factor of n-th triangular number.

%C Or, a(1) = 1, then the smallest nontrivial k (>1) which divides the sum of (next n) numbers from k+1 to k+n or smallest k > 1 that divides nk + n(n+1)/2. - _Amarnath Murthy_, Sep 22 2002. For example, a(7) = 4, which is the smallest nontrivial number that divides the sum 5+6+...+11, of 7 numbers.

%C a(n) = A020639(A000217(n)).

%H Zak Seidov, <a href="/A069901/b069901.txt">Table of n, a(n) for n = 1..10000</a>

%F a(4k-1) = a(4k) = 2.

%F From _Zak Seidov_, Jun 06 2013: (Start)

%F a(n) = 3 for n = {2, 5, 6, 9} + 12 k;

%F a(n) = 5 for n = {10, 25, 34, 49} + 60 k;

%F a(n) = 7 for n = {13, 97, 118, 133, 181, 202, 217, 238, 286, 301, 322, 406} + 420 k, etc. (end)

%e A000217(10) = 10*(10+1)/2 = 55 = 5*11, therefore a(10) = 5.

%t FactorInteger[#][[1,1]]&/@Accumulate[Range[100]] (* _Harvey P. Dale_, Apr 05 2014 *)

%Y Cf. A069902, A069903, A069904.

%K nonn

%O 1,2

%A _Reinhard Zumkeller_, Apr 10, 2002

%E Edited by _N. J. A. Sloane_, Sep 06 2008 at the suggestion of _Franklin T. Adams-Watters_

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Last modified March 29 15:16 EDT 2020. Contains 333107 sequences. (Running on oeis4.)