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Search: id:A078730
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| A078730 |
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Sum of products of two successive divisors of n. |
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+0
2
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| 0, 2, 3, 10, 5, 26, 7, 42, 30, 62, 11, 116, 13, 114, 93, 170, 17, 242, 19, 280, 171, 266, 23, 476, 130, 366, 273, 528, 29, 713, 31, 682, 399, 614, 285, 1070, 37, 762, 549, 1150, 41, 1342, 43, 1264, 873, 1106, 47, 1916, 350, 1562, 921, 1752, 53, 2186, 665, 2166, 1143
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OFFSET
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1,2
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COMMENT
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a(n) = dot_product (d_1,d_2,...,d_(tau(n)-1))*(d_2,d_3,...d_tau(n)), where d_1<d_2<...<d_tau(n), is increasing sequence of divisors of n. a(10) = dot_product (1,2,5)*(2,5,10) = 2+10+50 = 62.
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MATHEMATICA
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f[n_] := Module[{d, l, s, i}, d = Divisors[n]; l = Length[d]; s = 0; For[i = 1, i < l, i++, s = s + d[[i + 1]]*d[[i]]]; s]; Table[ f[n], {n, 1, 100}]
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CROSSREFS
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Cf. A078713(n) = 2*A001157(n)-2*a(n)-n^2-1.
Sequence in context: A112417 A139693 A064946 this_sequence A163767 A128531 A123167
Adjacent sequences: A078727 A078728 A078729 this_sequence A078731 A078732 A078733
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KEYWORD
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nonn
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AUTHOR
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Vladeta Jovovic (vladeta(AT)eunet.rs), Dec 20 2002
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