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A140269
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Numbers n such that gcd((largest divisor of n that is <= sqrt(n)), (smallest divisor of n that is >=sqrt(n))) > 1.
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2
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4, 8, 9, 16, 18, 24, 25, 27, 32, 36, 48, 49, 50, 54, 60, 64, 75, 80, 81, 96, 98, 100, 108, 112, 120, 121, 125, 128, 135, 140, 144, 147, 150, 160, 162, 168, 169, 180, 189, 192, 196, 200, 216, 224, 225, 242, 243, 245, 250, 252, 256, 264, 270, 280, 288, 289, 294
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OFFSET
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1,1
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COMMENTS
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G. Tenenbaum proved that this sequence has density 0 over the integers (théorème 3). - Michel Marcus, Nov 26 2012
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LINKS
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EXAMPLE
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The divisors of 48 are 1,2,3,4,6,8,12,16,24,48. The middle two of these divisors are 6 and 8, which are not coprime. So 48 is included in this sequence.
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MATHEMATICA
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okQ[n_] := (dd = Divisors[n]; s = Sqrt[n]; d1 = Select[dd, # <= s &] // Last; d2 = Select[dd, # >= s &] // First; !CoprimeQ[d1, d2]); Select[Range[300], okQ] (* Jean-François Alcover, Dec 27 2012 *)
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PROG
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(PARI) precdiv(n, k) = k=floor(k); while(k>0, if(n%k==0, return(k)); k--)
in140269(n) = local(d); d=precdiv(n, sqrt(n)); gcd(d, n\d)>1
(PARI) is(n)=if(issquare(n), n, my(d=divisors(n)); gcd(d[#d\2], d[#d\2+1]))>1 \\ Charles R Greathouse IV, Nov 26 2012
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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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