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Search: id:A099501
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| A099501 |
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Least number of members in a subset of integers in the interval [n^2+1, (n+1)^2-1] whose product is twice a square. |
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+0
6
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| 1, 1, 3, 1, 1, 2, 1, 1, 1, 2, 1, 1, 3, 1, 1, 1, 2, 1, 1, 3, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 3, 1, 1, 2, 1, 1, 1, 3, 1, 1, 2, 1, 1, 1, 3, 1, 1, 2, 1, 1, 1, 3, 1, 1, 2, 1, 1, 2
(list; graph; listen)
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OFFSET
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1,3
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COMMENT
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Granville and Selfridge conjecture that a(n) <= 3 for all n.
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LINKS
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Andrew Granville and John Selfridge, Product of integers in an interval, modulo squares (pdf), Electronic Journal of Combinatorics, Volume 8(1), 2001.
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EXAMPLE
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a(5) = 1 because the interval [26,35] contains two sets of such integers ({32} and {27,28,30,35}) and the smallest set has 1 member.
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MATHEMATICA
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Needs["DiscreteMath`Combinatorica`"]; Table[lst=Range[n^2+1, (n+1)^2-1]; x=Times@@lst; {p, e}=Transpose[FactorInteger[x]]; Do[If[e[[i]]==1 && p[[i]]!=2, lst=DeleteCases[lst, _?(Mod[ #, p[[i]]]==0&)]], {i, Length[p]}]; k=1; found=False; While[ !found && k<=Length[lst], m=0; While[ !found && m<Binomial[Length[lst], k], ss=UnrankKSubset[m, k, lst]; x=Times@@ss; If[Mod[x, 2]==0 && IntegerQ[Sqrt[x/2]], found=True]; m++ ]; k++ ]; (*Print[{n, ss, x}]; *)Length[ss], {n, 150}]
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CROSSREFS
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Cf. A099500 (number of subsets), A099502 (n such that a(n)=3).
Sequence in context: A070989 A066827 A016467 this_sequence A089762 A030347 A010275
Adjacent sequences: A099498 A099499 A099500 this_sequence A099502 A099503 A099504
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KEYWORD
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nonn
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AUTHOR
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T. D. Noe (noe(AT)sspectra.com), Oct 20 2004
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