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%I #14 Mar 13 2013 18:53:04
%S 6,10,15,20,21,28,35,36,45,55,56,66,70,78,84,91,105,126,136,153,165,
%T 171,190,220,231,252,253,276,286,300,325,330,351,364,378,406,435,455,
%U 462,465,495,496,528,560,561,595,630,666,680,703,715,741,780,792,816
%N Numbers occurring exactly in 2 rows of Pascal's triangle.
%C A059233(a(n)) = 2.
%H Reinhard Zumkeller and T. D. Noe, <a href="/A182237/b182237.txt">Table of n, a(n) for n = 1..1000</a> (first 100 terms from Reinhard Zumkeller)
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Singmaster's_conjecture">Singmaster's conjecture</a>
%H <a href="/index/Pas#Pascal">Index entries for triangles and arrays related to Pascal's triangle</a>
%t nn = 1000; t = Table[s = {}; k = 1; While[k++; b = Binomial[n, k]; k <= n/2 && b <= nn, AppendTo[s, b]]; s, {n, nn}]; t2 = Select[t, Length[#] > 0 &]; Transpose[Select[Tally[Sort[Flatten[t2]]], #[[2]] == 1 &]][[1]] (* _T. D. Noe_, Mar 13 2013 *)
%o (Haskell)
%o import Data.List (elemIndices)
%o a182237 n = a182237_list !! (n-1)
%o a182237_list = map (+ 2 ) $ elemIndices 2 a059233_list
%Y Cf. A098565.
%K nonn
%O 1,1
%A _Reinhard Zumkeller_, Dec 24 2012
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