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%I #5 Mar 30 2012 18:58:10
%S 2,3,2,3,4,5,4,3,6,4,3,5,7,4,6,5,5,6,5,4,10,8,7,5,9,7,11,4,9,6,4,6,8,
%T 5,10,6,7,5,11,6,13,10,16,8,6,7,6,5,9,9,8,7,10,11,13,6,14,9,8,6,16,8,
%U 10,7,17,9,15,5,12,10
%N Least k such that n divides s(k)-s(j) for some j satisfying 1<=j<k, where s(j)=j*(2^(j-1)).
%C See A204892 for a discussion and guide to related sequences.
%t s[n_] := s[n] = n*2^(n - 1); z1 = 250; z2 = 70;
%t Table[s[n], {n, 1, 30}] (* A001787 *)
%t u[m_] := u[m] = Flatten[Table[s[k] - s[j], {k, 2, z1}, {j, 1, k - 1}]][[m]]
%t Table[u[m], {m, 1, z1}] (* A205120 *)
%t v[n_, h_] := v[n, h] = If[IntegerQ[u[h]/n], h, 0]
%t w[n_] := w[n] = Table[v[n, h], {h, 1, z1}]
%t d[n_] := d[n] = First[Delete[w[n], Position[w[n], 0]]]
%t Table[d[n], {n, 1, z2}] (* A205121 *)
%t k[n_] := k[n] = Floor[(3 + Sqrt[8 d[n] - 1])/2]
%t m[n_] := m[n] = Floor[(-1 + Sqrt[8 n - 7])/2]
%t j[n_] := j[n] = d[n] - m[d[n]] (m[d[n]] + 1)/2
%t Table[k[n], {n, 1, z2}] (* A205122 *)
%t Table[j[n], {n, 1, z2}] (* A205123 *)
%t Table[s[k[n]], {n, 1, z2}] (* A205124 *)
%t Table[s[j[n]], {n, 1, z2}] (* A205125 *)
%t Table[s[k[n]] - s[j[n]], {n, 1, z2}] (* A205126 *)
%t Table[(s[k[n]] - s[j[n]])/n, {n, 1, z2}] (* A205127 *)
%Y Cf. A001787, A204892, A205127.
%K nonn
%O 1,1
%A _Clark Kimberling_, Jan 25 2012
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