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A122937 3-Round numbers: numbers n such that every number less than n and relatively prime to n has at most three prime factors (counting multiplicities). 3
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 84, 90, 96, 102, 108, 114, 120, 126, 132, 138, 144, 150, 156, 162, 168, 174, 180, 186, 192 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This sequence, for r=3 prime factors, is finite. Maillet proved that such sequences are finite for any fixed r. The case r=1 is A048597; case r=2 is A122936.

REFERENCES

Dickson, History of the Theory of Numbers, Vol. I, Chelsea, New York, 1952, p. 134.

LINKS

T. D. Noe, Table of n, a(n) for n=1..265 (complete sequence)

MATHEMATICA

Omega[n_] := If[n==1, 0, Plus@@(Transpose[FactorInteger[n]][[2]])]; nn=60060; r=3; moreThanR=Select[Range[nn], Omega[ # ]>r&]; lst={1}; Do[s=Select[Range[n], GCD[n, # ]==1&]; If[Intersection[s, moreThanR]=={}, AppendTo[lst, n]], {n, 2, nn}]; lst

CROSSREFS

Cf. A048597 (very round numbers), A051250, A089016 (largest n-round number).

Sequence in context: A115847 A204315 A032966 * A246076 A246082 A060340

Adjacent sequences:  A122934 A122935 A122936 * A122938 A122939 A122940

KEYWORD

fini,full,nonn

AUTHOR

T. D. Noe, Sep 21 2006

STATUS

approved

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Last modified July 15 20:00 EDT 2019. Contains 325056 sequences. (Running on oeis4.)