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A140747
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a(n) is the number of divisors of n that are coprime to the next larger divisor of n.
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2
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0, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 3, 1, 2, 2, 1, 1, 2, 1, 2, 2, 2, 1, 3, 1, 2, 1, 2, 1, 4, 1, 1, 2, 2, 2, 3, 1, 2, 2, 3, 1, 3, 1, 2, 3, 2, 1, 3, 1, 2, 2, 2, 1, 2, 2, 3, 2, 2, 1, 5, 1, 2, 3, 1, 2, 3, 1, 2, 2, 4, 1, 4, 1, 2, 2, 2, 2, 3, 1, 3, 1, 2, 1, 5, 2, 2, 2, 2, 1, 5, 2, 2, 2, 2, 2, 3, 1, 2, 2, 2, 1, 3, 1, 2, 4
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OFFSET
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1,6
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LINKS
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FORMULA
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EXAMPLE
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The divisors of 20 are 1,2,4,5,10,20. 1 is coprime to 2. 2 is not coprime to 4. 4 is coprime to 5. 5 is not coprime to 10. And 10 is not coprime to 20. So there are 2 divisors, 1 and 4, that are each coprime to the next higher divisor. So a(20) = 2.
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PROG
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(PARI) A140747(n) = { my(s=0, pd=0); fordiv(n, d, s += (pd&&(1==gcd(d, pd))); pd = d); s; }; \\ (Based on the fact that in PARI, if the first argument of fordiv is an integer, divisors are sorted by increasing size.) Antti Karttunen, Aug 12 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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