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Search: id:A122181
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| A122181 |
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Numbers n that can be written as n = x*y*z with 1<x<y<z (A122180(n)>0). |
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+0
2
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| 24, 30, 36, 40, 42, 48, 54, 56, 60, 64, 66, 70, 72, 78, 80, 84, 88, 90, 96, 100, 102, 104, 105, 108, 110, 112, 114, 120, 126, 128, 130, 132, 135, 136, 138, 140, 144, 150, 152, 154, 156, 160, 162, 165, 168, 170, 174, 176, 180, 182, 184, 186, 189, 190, 192, 195
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Equivalently, numbers n with at least 7 divisors (A000005(n)>6). Equivalently, numbers n with at least 5 proper divisors (A070824(n)>4). Equivalently, numbers n such that i) n has at least three distinct prime factors (A000977), ii) n has two distinct prime factors and four or more total prime factors (n=p^j*q^k, p,q prime, j+k>=4), or iii) n=p^k, a perfect power (A001597) but restricted to prime p and k>=6 [=1+2+3] (some terms of A076470).
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EXAMPLE
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a(1) = 24 = 2*3*4, a product of three distinct proper divisors (omega(24)=2, bigomega(24)=4).
a(2) = 30 = 2*3*5, a product of three distinct prime factors (omega(30)=3).
a(10) = 64 = 2*4*8 [= 2^1*2^2*2^3] (omega(64)=1, bigomega(64)=6).
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PROGRAM
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(PARI) for(n=1, 300, if(numdiv(n)>6, print1(n, ", ")) for(n=1, 300, if( (omega(n)==1 && bigomega(n)>5) || (omega(n)==2 && bigomega(n)>3) || (omega(n)>2), print1(n, ", ")))
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CROSSREFS
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Cf. A122179, A122180, A000005, A070824, A000977, A001221, A001222, A001597, A076470.
Sequence in context: A048260 A114635 A077969 this_sequence A167758 A075422 A098030
Adjacent sequences: A122178 A122179 A122180 this_sequence A122182 A122183 A122184
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KEYWORD
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nonn
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AUTHOR
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Rick L. Shepherd (rshepherd2(AT)hotmail.com), Aug 24 2006
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