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A141113
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Positive integers k such that d(d(k)) divides k, where d(k) is the number of divisors of k.
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4
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1, 2, 4, 6, 12, 15, 16, 20, 21, 24, 27, 28, 32, 33, 36, 39, 40, 44, 48, 51, 52, 56, 57, 60, 64, 68, 69, 72, 76, 80, 84, 87, 88, 90, 92, 93, 96, 104, 108, 111, 112, 116, 120, 123, 124, 126, 128, 129, 132, 136, 141, 144, 148, 150, 152, 156, 159, 164, 172, 176, 177, 180
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OFFSET
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1,2
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LINKS
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EXAMPLE
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28 has 6 divisors and 6 has 4 divisors. 4 divides 28, so 28 is in the sequence.
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MAPLE
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with(numtheory): a:=proc(n) if `mod`(n, tau(tau(n))) = 0 then n else end if end proc: seq(a(n), n=1..200); # Emeric Deutsch, Jun 05 2008
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MATHEMATICA
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Select[Range[200], Divisible[#, DivisorSigma[0, DivisorSigma[0, #]]]&] (* Harvey P. Dale, Feb 05 2012 *)
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PROG
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(PARI) is(k) = k%numdiv(numdiv(k)) == 0; \\ Jinyuan Wang, Feb 19 2019
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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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