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A072749
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Count of factors of 5 in the first n squarefree numbers.
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5
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0, 0, 0, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10, 11, 11, 11, 11, 12, 12, 12, 12, 13, 13, 13, 13, 13, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14
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OFFSET
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1,7
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LINKS
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Gheorghe Coserea, Table of n, a(n) for n = 1..10000
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FORMULA
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a(n) ~ n/6. - Amiram Eldar, Feb 24 2021
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EXAMPLE
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The first 10 squarefree numbers are: 1, 2, 3, 5, 6=2*3, 7, 10=2*5, 11, 13 and 14=2*7: 5 and 10 are divisible by 5, therefore a(10)=2.
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MATHEMATICA
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Accumulate[If[Divisible[#, 5], 1, 0]&/@Select[Range[150], SquareFreeQ]] (* Harvey P. Dale, Oct 17 2013 *)
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PROG
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(PARI)
n = 89; k = 0; bag = List(); a = vector(n);
until(n == 0, k++; if (issquarefree(k), listput(bag, k); n--));
for (i=2, #bag, a[i] = a[i-1] + (bag[i] % 5 == 0));
print(a); \\ Gheorghe Coserea, Aug 22 2015
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CROSSREFS
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Cf. A005117, A072747, A072748, A072750, A072751.
Sequence in context: A000523 A124156 A324965 * A066490 A156685 A124230
Adjacent sequences: A072746 A072747 A072748 * A072750 A072751 A072752
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KEYWORD
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nonn,changed
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AUTHOR
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Reinhard Zumkeller, Jul 08 2002
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EXTENSIONS
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Name clarified by Gheorghe Coserea, Aug 22 2015
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STATUS
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approved
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