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A080982 Smallest k such that the k-th triangular number has n^2 as divisor. 4

%I

%S 1,7,8,31,24,8,48,127,80,24,120,63,168,48,99,511,288,80,360,224,98,

%T 120,528,512,624,168,728,735,840,224,960,2047,242,288,49,1215,1368,

%U 360,675,1024,1680,440,1848,1088,324,528,2208,512,2400,624,288,1183,2808,728

%N Smallest k such that the k-th triangular number has n^2 as divisor.

%C a(2^k) = 2^(2*k+1)-1; a(m)=m^2-1 and for odd prime powers m;

%C A080983(n)=A000217(a(n)).

%H Reinhard Zumkeller, <a href="/A080982/b080982.txt">Table of n, a(n) for n = 1..1000</a>

%o (Haskell)

%o import Data.List (findIndex)

%o import Data.Maybe (fromJust)

%o a080982 n = (+ 1) $ fromJust $

%o findIndex ((== 0) . (`mod` (n ^ 2))) $ tail a000217_list

%o -- _Reinhard Zumkeller_, Mar 23 2013

%Y Cf. A011772.

%K nonn

%O 1,2

%A _Reinhard Zumkeller_, Feb 26 2003

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Last modified May 19 04:06 EDT 2019. Contains 323377 sequences. (Running on oeis4.)