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Search: id:A163516
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| A163516 |
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a(n) = floor( sum_{x=2..n} x/log(x) ). |
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1
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| 0, 2, 5, 8, 11, 14, 18, 22, 26, 30, 35, 40, 45, 50, 56, 61, 67, 74, 80, 87, 94, 101, 108, 116, 123, 131, 140, 148, 157, 165, 175, 184, 193, 203, 213, 223, 233, 243, 254, 265, 276, 287, 299, 310, 322, 334, 346, 359, 371, 384, 397, 410, 424, 437, 451, 465, 479, 493
(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) closely approximates the number of primes < n^2, that is A038107(n) = Pi(n^2).
In fact, the sum is as good as Li(n^2). For n = 10^9,
a(n) = 24739954333817884.
Pi(n^2) = 24739954287740860 = A006880(18).
Li(n^2) = 24739954309690415 = A057754(18) = A089896(18).
R(n^2) = 24739954284239494 = A057793(18).
Now x/(log(x)-1) is a much better approximation of Pi(x) than x/log(x).
10^18/(log(10^18)-1)=24723998785919976 and 10^18/log(10^18)=24127471216847323.
Ironically though, a(n) = sum(x=2,n,x/(log(x)-1) is way off Pi(n^2), see A058290.
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FORMULA
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a(10^n) = A163521(n).
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EXAMPLE
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For n = 10, floor(sum(x=2,n,x/log(x))) = 30, the the 10-th entry.
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PROGRAM
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(PARI) nthsum(n) = for(j=1, n, print1(floor(sum(x=2, j, x/log(x)))", "));
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CROSSREFS
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Sequence in context: A016789 A165334 A135677 this_sequence A000093 A070214 A031210
Adjacent sequences: A163513 A163514 A163515 this_sequence A163517 A163518 A163519
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KEYWORD
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nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)hotmail.com), Jul 30 2009
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EXTENSIONS
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Offset corrected, definition detailed, 7 references to other sequences added by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Aug 29 2009
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