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Search: id:A130698
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| A130698 |
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Numbers n such that the sum of the divisors function of all integers up to n is a square. |
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+0
1
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OFFSET
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1,2
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COMMENT
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It is not known if this sequence is infinite.
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FORMULA
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n such that sigma(1)+sigma(2)+...+sigma(n)=x^2 with some integer x.
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EXAMPLE
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a(2)=2 since sigma(1)=1^2 and sigma(1)+sigma(2)=2^2.
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MATHEMATICA
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L=0; For[j = 1, j < 1000000, j++, L = L + DivisorSigma[1, j]; If[L == Floor[Sqrt[L]]^2, Print[j, " ", Floor[Sqrt[L]]]]]
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CROSSREFS
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Adjacent sequences: A130695 A130696 A130697 this_sequence A130699 A130700 A130701
Sequence in context: A059703 A041337 A139844 this_sequence A123005 A142477 A119112
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KEYWORD
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nonn
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AUTHOR
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Florian Luca (fluca(AT)matmor.unam.mx), Jul 11 2007
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