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A243285
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Number of integers 1 <= k <= n which are not divisible by the square of their largest noncomposite divisor.
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8
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0, 1, 2, 2, 3, 4, 5, 5, 5, 6, 7, 8, 9, 10, 11, 11, 12, 12, 13, 14, 15, 16, 17, 18, 18, 19, 19, 20, 21, 22, 23, 23, 24, 25, 26, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 38, 38, 39, 40, 41, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 50, 51, 52, 53, 54, 55, 56, 57
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OFFSET
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1,3
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COMMENTS
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a(n) tells how many natural numbers <= n there are which are not divisible by the square of their largest noncomposite divisor.
The largest noncomposite divisor of 1 is 1 itself, and 1 is divisible by 1^2, thus 1 is not included in the count, and a(1)=0.
The "largest noncomposite divisor" for any integer > 1 means the same thing as the largest prime divisor, and thus we are counting the terms of A102750 (Numbers n such that square of largest prime dividing n does not divide n).
Thus this is the partial sums of the characteric function for A102750.
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LINKS
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FORMULA
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For all n, a(A102750(n)) = n, thus this sequence works also as an inverse function for the injection A102750.
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EXAMPLE
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For n = 9, there are numbers 2, 3, 5, 6 and 7 which are not divisible by the square of their largest prime factor, while 1 is excluded (no prime factors) and 4 and 8 are divisible both by 2^2 and 9 is divisible by 3^2. Thus a(9) = 5.
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MATHEMATICA
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ndsQ[n_]:=Mod[n, Max[Select[Divisors[n], !CompositeQ[#]&]]^2]!=0; Accumulate[Table[If[ ndsQ[n], 1, 0], {n, 80}]] (* Harvey P. Dale, Oct 14 2023 *)
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PROG
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(Python)
from sympy import primefactors
def a243285(n): return 0 if n==1 else sum([1 for k in range(2, n + 1) if k%(primefactors(k)[-1]**2)!=0]) # Indranil Ghosh, Jun 15 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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STATUS
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approved
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