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A115958
Numbers n having exactly 4 distinct prime factors, the largest of which is greater than or equal to sqrt(n) (i.e., sqrt(n)-rough numbers with exactly 4 distinct prime factors).
6
930, 1110, 1230, 1290, 1410, 1590, 1770, 1806, 1830, 1974, 2010, 2130, 2190, 2226, 2370, 2478, 2490, 2562, 2670, 2814, 2910, 2982, 3030, 3066, 3090, 3210, 3270, 3318, 3390, 3486, 3660, 3738, 3810, 3930, 4020, 4074, 4110, 4170, 4242, 4260, 4326, 4380
OFFSET
1,1
LINKS
EXAMPLE
3660 is in the sequence because it has 4 distinct prime factors (2, 3, 5 and 61) and 61 > sqrt(3660).
MAPLE
with(numtheory): a:=proc(n) if nops(factorset(n))=4 and factorset(n)[4]^2>=n then n else fi end: seq(a(n), n=1..4500);
MATHEMATICA
pf4Q[n_]:=Module[{f=FactorInteger[n]}, Length[f]==4 && f[[-1, 1]] >= Sqrt[ n]]; Select[Range[5000], pf4Q] (* Harvey P. Dale, Sep 13 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Feb 02 2006
STATUS
approved