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A080194
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7-smooth numbers which are not 5-smooth.
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13
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7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 84, 98, 105, 112, 126, 140, 147, 168, 175, 189, 196, 210, 224, 245, 252, 280, 294, 315, 336, 343, 350, 378, 392, 420, 441, 448, 490, 504, 525, 560, 567, 588, 630, 672, 686, 700, 735, 756, 784, 840, 875, 882, 896, 945, 980
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OFFSET
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1,1
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COMMENTS
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Numbers of the form 2^r*3^s*5^t*7^u with r, s, t >= 0, u > 0.
Multiples of 7 which are members of A002473. Or multiples of 7 with the largest prime divisor < 10.
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LINKS
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Table of n, a(n) for n=1..55.
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EXAMPLE
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28 = 2^2*7 is a term but 30 = 2*3*5 is not.
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MATHEMATICA
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nn=1000; With[{s7=Select[Range[nn], Max[Transpose[FactorInteger[#]] [[1]]]<= 7&], s5=Select[Range[nn], Max[Transpose[FactorInteger[#]][[1]]]<=5&]}, Complement[s7, s5]] (* or *) Select[7*Range[150], Max[Select[Divisors[#], PrimeQ]]<10&] (* Harvey P. Dale, Jan 05 2013 *)
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PROG
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(PARI) {m=1010; z=[]; for(r=0, floor(log(m)/log(2)), a=2^r; for(s=0, floor(log(m/a)/log(3)), b=a*3^s; for(t=0, floor(log(m/b)/log(5)), c=b*5^t; for(u=1, floor(log(m/c)/log(7)), z=concat(z, c*7^u))))); z=vecsort(z); for(i=1, length(z), print1(z[i], ", "))}
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CROSSREFS
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Cf. A002473, A051037.
Cf. A085125, A085126, A085127, A085128, A085129, A085131, A085132.
Sequence in context: A182341 A008589 A085130 * A206717 A043393 A028437
Adjacent sequences: A080191 A080192 A080193 * A080195 A080196 A080197
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KEYWORD
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easy,nonn
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AUTHOR
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Klaus Brockhaus, Feb 10 2003
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STATUS
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approved
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