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A005236 Barriers for omega(n): numbers n such that, for all m < n, m + omega(m) <= n.
(Formerly M0501)
3
2, 3, 4, 5, 6, 8, 9, 10, 12, 14, 17, 18, 20, 24, 26, 28, 30, 33, 38, 42, 48, 50, 54, 60, 65, 74, 82, 84, 90, 98, 102, 108, 110, 114, 126, 129, 138, 150, 164, 168, 174, 180, 194, 198, 228, 234, 244, 252, 258, 264, 270, 290, 294, 318, 348, 354, 360, 384, 390, 402
(list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
omega(m) is the number of distinct prime factors of m.
REFERENCES
R. K. Guy, Unsolved Problems in Number Theory, B8.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
Paul Erdős, Some Unconventional Problems in Number Theory, Mathematics Magazine, Vol. 52, No. 2, Mar., 1979, pp. 67-70. See Problem 4, p. 68.
Paul Erdős, Some unconventional problems in number theory, Acta Mathematica Hungarica, 33(1):71-80, 1979.
EXAMPLE
1 + omega(1) = 1, 2 + omega(2) = 3, 3 + omega(3) = 4, 4 + omega(4) = 5, 5 + omega(5) = 6.
Thus we have verified that m + omega(m) < 6 for m < 6, so 6 is in the sequence.
But since 6 + omega(6) = 8 > 7, 7 is not in the sequence.
MATHEMATICA
omegaBarrierQ[n_] := (For[m = 1, m < n, m++, If[m + PrimeNu[m] > n, Return[False]]]; True); Select[Range[2, 500], omegaBarrierQ] (* Jean-François Alcover, Feb 03 2015 *)
PROG
(PARI) is(n)=for(k=1, log(n)\log(5), if(omega(n-k)>k, return(0))); n>1 \\ Charles R Greathouse IV, Sep 19 2012
(Haskell)
a005236 n = a005236_list !! (n-1)
a005236_list = filter (\x -> all (<= x) $ map a229109 [1..x-1]) [2..]
-- Reinhard Zumkeller, Sep 13 2013
CROSSREFS
Sequence in context: A164043 A172248 A082415 * A051250 A143071 A305759
KEYWORD
nonn,nice
AUTHOR
EXTENSIONS
More terms from John W. Layman
STATUS
approved

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Last modified September 18 11:24 EDT 2024. Contains 376000 sequences. (Running on oeis4.)