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Search: id:A071860
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| A071860 |
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Number of k 1<=k<=n such that sigma(k) is odd. |
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+0
2
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| 1, 2, 2, 3, 3, 3, 3, 4, 5, 5, 5, 5, 5, 5, 5, 6, 6, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 11, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13, 13, 13, 13, 13, 13, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 14, 15
(list; graph; listen)
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OFFSET
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2,2
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COMMENT
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a(n) = partial sums of A053866(n-1) and A093709(n-1). [From Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Oct 18 2009]
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REFERENCES
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R. Crandall, C. Pomerance, Prime numbers: a computational perspective. Springer-Verlag, New York, 2001, p. 52.
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FORMULA
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a(n) = floor( C * sqrt(n) ) +- 1, 0 with C = 1+1/sqrt(2) = 1, 707...
a(n) = floor(sqrt(n)) + floor(sqrt(n/2)). (Crandall, Pomerance). - Franz Vrabec (franz.vrabec(AT)aon.at), Jun 24 2006
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PROGRAM
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(PARI) for(n=1, 100, print1(sum(i=1, n, if(sigma(i)%2, 1, 0)), ", "))
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CROSSREFS
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Cf. A028982.
Sequence in context: A108356 A055656 A078571 this_sequence A004788 A034584 A035430
Adjacent sequences: A071857 A071858 A071859 this_sequence A071861 A071862 A071863
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KEYWORD
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easy,nonn
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AUTHOR
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Benoit Cloitre (benoit7848c(AT)orange.fr), Jun 09 2002
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