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A085987 Product of exactly four primes, three of which are distinct (p^2*q*r). 24
60, 84, 90, 126, 132, 140, 150, 156, 198, 204, 220, 228, 234, 260, 276, 294, 306, 308, 315, 340, 342, 348, 350, 364, 372, 380, 414, 444, 460, 476, 490, 492, 495, 516, 522, 525, 532, 550, 558, 564, 572, 580, 585, 620, 636, 644, 650, 666, 693, 708, 726 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A014613 is completely determined by A030514, A065036, A085986, A085987 and A046386 since p(4) = 5. (cf. A000041). More generally, the first term of sequences which completely determine the k-almost primes can be found in A036035 (a resorted version of A025487).

Subsequence of A178212. - Reinhard Zumkeller, May 24 2010

A050326(a(n)) = 4. - Reinhard Zumkeller, May 03 2013

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

Index to sequences related to prime signature

EXAMPLE

a(1) = 60 since 60 = 2*2*3*5 and has three distinct prime factors.

MATHEMATICA

f[n_]:=Sort[Last/@FactorInteger[n]]=={1, 1, 2}; Select[Range[2000], f] (* Vladimir Joseph Stephan Orlovsky, May 03 2011 *)

PROG

(PARI) list(lim)=my(v=List(), t, x, y, z); forprime(p=2, lim^(1/4), t=lim\p^2; forprime(q=p+1, sqrtint(t), forprime(r=q+1, t\q, x=p^2*q*r; y=p*q^2*r; listput(v, x); if(y<=lim, listput(v, y); z=p*q*r^2; if(z<=lim, listput(v, z)))))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Jul 15 2011

(PARI) is(n)=vecsort(factor(n)[, 2]~)==[1, 1, 1, 2] \\ Charles R Greathouse IV, Oct 19 2015

CROSSREFS

Cf. A000007, A000040, A001248, A006881, A030078, A054753, A007304, A050997, A046387, A036035, A086974.

Sequence in context: A182855 A009129 A174292 * A086974 A099831 A138604

Adjacent sequences:  A085984 A085985 A085986 * A085988 A085989 A085990

KEYWORD

nonn

AUTHOR

Alford Arnold, Jul 08 2003

EXTENSIONS

More terms from Reinhard Zumkeller, Jul 25 2003

STATUS

approved

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Last modified September 22 22:42 EDT 2018. Contains 315270 sequences. (Running on oeis4.)