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Search: id:A018805
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| A018805 |
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Number of elements in the set {(x,y): 1<=x,y<=n, 1=gcd(x,y)}. |
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+0
24
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| 1, 3, 7, 11, 19, 23, 35, 43, 55, 63, 83, 91, 115, 127, 143, 159, 191, 203, 239, 255, 279, 299, 343, 359, 399, 423, 459, 483, 539, 555, 615, 647, 687, 719, 767, 791, 863, 899, 947, 979, 1059, 1083, 1167, 1207, 1255, 1299, 1391, 1423, 1507, 1547, 1611, 1659, 1763
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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a(n) = 2*A015614(n) + 1. - Reinhard Zumkeller (reinhard.zumkeller(AT)lhsystems.com), Apr 08 2006
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REFERENCES
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Cai, Jin-Yi; Bach, Eric. On testing for zero polynomials by a set of points with bounded precision, Theoret. Comput. Sci. 296 (2003), no. 1, 15-25. MR1965515 (2004m:68279).
S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 110-112.
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954.
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LINKS
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Pieter Moree, Counting carefree couples
Eric Weisstein's World of Mathematics, Carefree Couple
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FORMULA
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a(n) = 2 ( Sum phi(j), j=1..n ) - 1; a(n) = n^2 - Sum a([ n/j ]), j=2..n.
a(n) ~ (1/Zeta(2)) * n^2 = (6/pi^2) * n^2 as n goes to infinity (zeta is the Riemann zeta function and the constant 6/pi^2 is 0.607927...). - Ahmed Fares (ahmedfares(AT)my-deja.com), Jul 18 2001
a(n)=sum(k=1, n, mu(k)*floor(n/k)^2) - Benoit Cloitre (benoit7848c(AT)orange.fr), May 11 2003
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MATHEMATICA
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FoldList[ Plus, 1, 2 Array[ EulerPhi, 60, 2 ] ]
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CROSSREFS
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Cf. A100613 (gcd > 1), A071778 (triples).
Sequence in context: A092109 A117991 A118260 this_sequence A135932 A105876 A105888
Adjacent sequences: A018802 A018803 A018804 this_sequence A018806 A018807 A018808
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KEYWORD
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nonn
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AUTHOR
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David W. Wilson (davidwwilson(AT)comcast.net)
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EXTENSIONS
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Mma Program Aug 15 1997 (Olivier Gerard).
More terms from Reinhard Zumkeller (reinhard.zumkeller(AT)lhsystems.com), Apr 08 2006
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