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A071604
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a(n) is the number of 7-smooth numbers <= n.
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2
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1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 11, 11, 12, 13, 14, 14, 15, 15, 16, 17, 17, 17, 18, 19, 19, 20, 21, 21, 22, 22, 23, 23, 23, 24, 25, 25, 25, 25, 26, 26, 27, 27, 27, 28, 28, 28, 29, 30, 31, 31, 31, 31, 32, 32, 33, 33, 33, 33, 34, 34, 34, 35, 36, 36, 36, 36, 36, 36, 37, 37, 38
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OFFSET
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1,2
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COMMENTS
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A 7-smooth number is a number of the form 2^x*3^y*5^z*7^u, (x,y,z,u) >= 0.
In other words, a 7-smooth number is a number with no prime factor greater than 7. - Peter Munn, Nov 20 2021
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LINKS
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FORMULA
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a(n) = Card{ k | A002473 (k) <= n }.
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EXAMPLE
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a(11) = 10 as there are 10 7-smooth numbers <= 11. Namely 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. - David A. Corneth, Apr 19 2021
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PROG
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(PARI) for(n=1, 100, print1(sum(k=1, n, if(sum(i=5, n, if(k%prime(i), 0, 1)), 0, 1)), ", "))
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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