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 A067247 Number of difference sets of subsets of {1,2,...,n}, i.e., the size of {A-A : A \subset [n] }, where A-A={a_i-a_j : a_i>a_j and a_i,a_j \in A}. 0
 1, 2, 4, 6, 10, 16, 25, 39, 63, 99, 158, 253, 402, 639, 1021, 1633, 2617, 4153, 6633, 10460, 16598, 26146, 41409, 64733, 102006, 159165 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 2^(floor(n/2)) <= a(n) <= 2^n. LINKS EXAMPLE a(4)=6 because {1}, {1,2}, {1,3}, {1,4}, {1,2,3} and {1,2,4} have difference sets \emptyset, {1}, {2}, {3}, {1,2}, {1,2,3}, respectively and all 2^4 subsets of {1,2,3,4} have one of these difference sets. MATHEMATICA SetToNumber = Compile[{{A, _Integer, 1}, {LP, _Integer}}, Plus @@ (2^Union[Flatten[Table[If[i > j, A[[i]] - A[[j]], 0], {j, LP}, {i, LP}]]])]; GetSetA = Compile[{{n, _Integer}}, Flatten[Position[IntegerDigits[n, 2], 1]]]; DS[n_] := Module[{LP, A}, A = GetSetA[n]; LP = Length[A]; SetToNumber[A, LP]]; newfset[d_] := Union[Table[DS[n], {n, 2^(d - 1) + 1, 2^d - 1, 2}]]; newf[d_] := newf[d] = Length[newfset[d]]; a[2] = 2; a[d_] := a[d] = newf[d] + a[d - 1]; CROSSREFS Sequence in context: A305498 A028445 A006305 * A017985 A327474 A028488 Adjacent sequences:  A067244 A067245 A067246 * A067248 A067249 A067250 KEYWORD nonn,more AUTHOR Kevin O'Bryant, Mar 10 2002 STATUS approved

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Last modified December 11 12:33 EST 2019. Contains 329916 sequences. (Running on oeis4.)